White Canker
Tree & Shrub Disease
Our trees and shrubs are now experiencing a new and serious disease that is slowly weakening and killing them. This is a widespread disease, not limited to any particular geographic area. Little is known about it, since it only seemed to make its appearance around 2003. Yet its aggressive nature seems to overcome the natural defenses of our trees and shrubs.

How bad is it? Think of Dutch Elm disease, which has killed most of our elm trees. Also recall the American chestnut. Both of these much-loved trees are now virtually gone from our landscape. But this disease is worse, in that it seems to attack a wide variety trees and shrubs, not just a few selective species.

Disease Symptom Overview

Canker: A localized diseased or necrotic area on a plant part, especially on a trunk, branch, or twig of a woody plant, usually caused by fungi or bacteria.
This is a fungal disease.
It eats away at all interior parts of the tree or shrub, eventually replacing the tissue with white fungal material, which we call canker. Eventually the tree or shrub is consumed by this canker, and it dies, unable to transport food and water throughout its body. Externally, it may appear that lack of food or water is the reason for the decline./

Note: Because I'll frequently be referring to "tree" and "shrub" (and sometimes "plant") throughout this website, for brevity I'm going to use the generic term "plant" to refer to all three.

Pathogen: An infectious biological agent that causes disease or illness to its host.

Visual Disease Symptoms: Upon initial infection, you probably won't notice anything at all that is unusual about the plant. Most plants have a surprisingly strong tolerance to infectious agents, and will continue to look well even if there is an infection underway. Actually, it's the same with people - some people can be infected with a virus and it hardly affects them at all. And among plants, the tolerance to a particular pathogen also varies. Some plants are very susceptible and others fight off the infection very well. Again, just like people do.

With this fungal disease, the decline in plant health is only visually detectable after the fungus has made significant progress in consuming the tree tissue.

Overall Exterior Appearance: As this infection progresses, the overall appearance of the plant will give the impression that it is lacking water or nutrients. The plant will often have a "droopy" appearance. The casual observer will surmise that food, water, drought, wetness, heat, or cold is a major contributing factor.

Phloem: Inner bark tissue that transports food (sugars) from the leaves to other parts of the tree.
The bark: Like any successful organism, this fungus gravitates towards the major source of food in the plant - the phloem layer. The phloem layer lies just under the outer bark of trees and shrubs, and serves to transport food throughout the plant. The fungal "roots", called hypha, infiltrate the phloem and steal the nutrients. But these nutrients also support the bark in that they keep it healthy by suffusing it with chemicals which resist attack by other fungus and insects.

This fungal infection thus causes the bark to starve and decline in health. This unhealthy bark will look blacker than usual, and may give the general appearance that it is rotting. Sometimes this unhealthy bark will cause large bodies of lichen to move in.

Another consequence of this unhealthy bark is bark splitting. The unhealthy bark loses some of its normal elasticity, so that new bark pushing outward can't evenly distribute the growth, and hence a tear, or vertical fissure sometimes occurs that is several feet long, exposing the innards of the tree. These cracks can be several feet long and can sometimes bleed sap. The tree will then see this as a normal physical injury and try to heal this split. Interestingly, the new healing tissue will not have normal bark on it due to the infested phloem! However, one must be aware that these vertical cracks can also be caused by temperature extremes during cold weather.

The sick bark may also not adhere to the tree, and may simply fall off in pieces or chunks!

The particular bark symptoms mentioned above are very dependent upon the species of tree. Regardless, the bark is sick. Consequently, any bark mulch made from this bark will not have the nice aromatic smell of normal/healthy bark mulch. (Have you noticed that most bark mulch you've bought in the past 10 years or so smells moldy?)

Xylem: Core tissue that provides strength and transports water and minerals from the roots to the leaves. Sometimes called sapwood.
The leaves: As the plant is being poisoned and choked, the lack of essential nutrients being sent to the leaves causes them to grow abnormally. They can appear cupped or mottled. Furthermore, if this fungus weren't bad enough, it not only attacks the phloem, but it also attacks the plant's xylem, which is the inner part of the plant that transports water and minerals from the roots throughout the plant. This lack of water will cause the leaves to appear dry, lose their sheen, droop, and curl up. And if this isn't bad enough, this fungus also infiltrates the leaves. So when the poisoned and starved leaves die, they will often die from the leaf edges back to the stem.

Of course, a sick leaf that lacks sufficient water and nutrients can't properly defend itself against attacks by other pathogens and insects, so they may move in to feast on the weakened leaves.

An infected sick leaf is of little benefit to a plant, so the plant will sometimes shed them in an attempt to keep an infection from spreading, and to conserve water and food for the remaining leaves. The result is varying degrees of defoliation, sometimes making it seem like fall has arrived!

Anthracnose: Any of several diseases of plants caused by certain fungi and characterized by dead spots on the leaves, twigs, or fruits.

With other tree diseases, such as anthracnose, the tree can replace the dead leaves later that growing season, or in the next season. But in this case a lifeless appearing branch is simply dead, being filled with canker, and will never support leaves again. The entire tree may be killed in 3 to 10 years.

Again, trees vary in their strategy for dealing with pathogens, so none of these symptoms is definitive in identifying this fungal disease. I'll discuss a definitive way to identify this disease later on.

What the Experts Think this Disease is

Having lived and gardened at this location for over 40 years, I'd never seen an abrupt tree and shrub health decline as occurred around 2003. Trees and shrubs that had always been very healthy were now showing severe signs of disease - all at the same time! It seemed obvious to me that there must be some common underlying cause, so I started calling on the experts.

First I called on the University of New Hampshire country extension office. The fellow there was very nice, but had no idea what the problem was. I then went one step up and sent a sample in to the University of New Hampshire (UNH) plant diagnostic lab. They too were unable to identify it, but guessed it was some kind of root fungus.

Then I had a lawn care company representative examine the diseased trees and shrubs, and he guessed that it was anthracnose. He recommending deep root watering and fertilizing. But to me it didn't seem like anthracnose was affecting so many plants at once.

Next, I called in two federal foresters, whose diagnoses ran all over the place from bugs, to viruses, to anthracnose. Surprisingly, they didn't think this massive decline was abnormal!

I then sent in 8 leaf samples to the Cornell plant pathology lab. They diagnosed most of the samples as having anthracnose. The UNH plant website also said anthracnose was especially bad that year, and that the solution was to simply wait it out. So I tried that recommendation with my rapidly declining Kwanzan cherry tree. The tree continued its decline, and a few months later it totally died. That was strange, because anthracnose isn't known for killing trees.

And it wasn't just trees and shrubs in my yard. Neighbors had similar problems. In fact, I began to see similar symptoms on trees all around the state, and in neighboring states. This negated the argument that I must have been doing something to cause the problem, like over/under fertilizing, watering, disturbing the roots, etc.

The next year I gathered up about 24 diseased shrub and tree samples and sent them to the well-known Cornell plant pathology lab for analysis. Once again, a variety of diagnoses was returned.

In summary, the experts couldn't find a common cause for this disease. Meanwhile, as I continued to study the disease symptoms in greater and greater depth on a wider and wider range of trees and shrubs, I became more and more convinced that there was one pathogen that was the source of this massive decline.

My Guess as to What this Disease is

I should make one thing clear from the start - while many of the diseased trees and shrubs were diagnosed as having anthracnose, I am convinced that while they may indeed have had anthracnose, the anthracnose was there because of the weakened condition of the plant. I felt this weakening was due to a primary disease, one that plant pathology labs seem unable to find. I realize that these plant pathology labs disagree with this conclusion. Their view is that there is no disease known that attacks such a wide variety of trees and shrubs.

Phytophthora: (Fy-TOFF-thor-uh) A fungal-like water mold. For more information, see en.wikipedia.org/wiki/Phytophthora.
Armed with the range of disease symptoms I'd recorded, I did extensive searches on the Internet. I also studied a key plant pathology industry reference book called Diseases of Trees and Shrubs, Second Edition, by Sinclair and Lyon of Cornell University. They are recognized experts on plant diseases.

The pathogen that best matched the disease symptoms appeared to be a fungal-like organism called a Phytophthora. I wondered if it might be Phytophthora ramorum, which is a relatively new disease killing off many trees and shrubs, mainly in California. Its common name is Sudden Oak Death. So I had a lab specifically check for Phytophthora ramorum within some samples I sent in. The diagnosis came back negative.

But of all the Phytophthora species, the best species match seemed to be Phytophthora cactorum. P. cactorum grows under the bark, choking off all nourishment flowing up and down the tree bark. Furthermore, Sinclair and Lyon say it has a broad host range and a global distribution. Its symptom list is Decline, Dieback, Fruit rot, Root crown and/or collar rot, Stem canker, and Seedling decay.

So P. cactorum seemed like an excellent match. Was that really it? When I sent a sample of diseased bark from a red oak tree to the Cornell plant pathology lab, the diagnosis came back positive. But another diseased bark sample from another tree sent to the UNH plant pathology lab came back as "definitely not a Phytophthora".

To try to resolve the issue, I contacted a plant pathology lab on the west coast that specialized in Phytophthora diseases. They wanted a root sample. So I sent them a sample of the roots from the red oak that Cornell said was infected with Phytophthora. The west coast lab said they found no Phytophthora!

Confused? I certainly was. Consequently, I still don't have a scientific name for this disease.

Something else?
So if white canker doesn't seem to match a fungus or phytophthora, what can it possibly be? I've asked myself that question hundreds of times over the years, and I've gradually come up with what many would consider a crazy idea - it's a new type of fungal infection, neither a true fungus nor a phytophthora. My best guess is that white canker is a genetic cross of a fungus and white pine pollen. I say this because white canker fruiting bodies are virtually identical to white pine pollen and mature and disperse at about the same time of year. Also, these white canker fruiting bodies most densely populate the lower facing part of branch junctions within a foot or two of branch tips - just where the pine cones of white pine trees form! Furthermore, as the hundreds of photos within this website show over and over, white canker behaves like a true fungus in that it has abundant fruiting bodies associated with huge amounts of supporting fungal hypha.

Finally, as so many pictures on this website show, white canker seems to attack an endless number of shrubs, plants, and trees. No known fungus or phytophthora comes close to doing this, so white canker appears to be a class of pathogen that shrubs, plants, and trees have not evolved an immunity to, so it is probably very new and unique.

In spite of the fact that this guess seems to make logical sense, I'm fully aware that many specialists in plant disease will find this hypothesis too radical to believe. But I think they may come around when they do the same microscopic research I did. Ultimately, though, the unquestionable truth will appear when someone finally does a DNA analysis of white canker and compares the result to the DNA of other forms of microbial life.

Why I Named this Disease White Canker

Since it's awkward to keep referring to this disease as "this disease" within this document, I've decided to give it a temporary name that reflects its chief diagnostic characteristics. First and foremost, this disease shows itself as a fungal disease that produces cankers in plant tissue. A canker is a disease-like growth under the bark of a tree or plant. In fact, that's often the primary characteristic that is evident when looking over a tree or shrub with this disease. A second significant identifying disease characteristic is only evident when looking at the cankerous tissue within the internal plant tissue under a high-power (e.g., 400x) microscope. It is there that you can see the real damage being done. This damage is in the form of enormous amounts of white wax-like growths invading and consuming the plant tissue.

Given these two primary disease characteristics, it seems to make sense to call this disease White Canker, even though there is nothing white about it when seen with the unaided eye.

In time, researchers will no doubt determine exactly what this disease is, and assign a scientific name to it. In the meantime, white canker seems to best describe this disease with a minimum number of words.

General Diagnostic Methods

There are several methods that you can use to try to determine what disease or diseases are affecting your plants. They differ in the amount of time and cost involved, and in the diagnosis confidence. Some suggestions follow. I've tried them all.

Try an Internet search
There are many plant diseases out there. Your first step in diagnosis might be to examine the diseased plant for some key disease characteristics, and then do an Internet search for these characteristics. That's a relatively easy step, is fast, is cheap, and is what I initially did.

Call in a lawn, tree, and shrub specialist
It's the business of these commercial companies to diagnose and treat plant diseases. The diagnosis is generally free. They make their money in chemically treating the disease they find. Some diseases they identify can be relatively easy for the homeowner to treat themselves.

Bring a sample diseased plant to your county extension
States usually have extension offices that deal in plant diseases. Often, you can bring a sample of a diseased plant into them, and an expert there can identify the disease for you. Of course, it will take some time to drive there and back, but the service is free.

Try a Plant Pathology Diagnostic Lab
If you want one of the best diagnostic services available, you can send a sample of the diseased plant to a state plant pathology diagnostic lab. Many states have university labs that will perform this service for you. Costs vary.

For example, I've sent leaf and bark samples to both the UNH Plant Diagnostic Lab and the Cornell University Plant Disease Diagnostic Clinic.

One word of caution, though. If they diagnose a Phytophthora disease, they are unable to diagnose its particular species. Consequently, that might prevent you from choosing an appropriate chemical treatment. They tell me that species determination would require expensive and time-consuming lab tests.

On the other hand, since Phytophthora ramorum is a serious and rapidly spreading disease, there is a test specifically for it (at the Cornell lab, at least), and the test is free.

Do your Own Testing
The late Dr. Alex Shigo, a renowned expert on trees, suggested that we carefully observe the trees, to see what they had to say. I really liked that advice, so I spent many hours outside looking at diseased trees. Then I spent many hours inside examining the details of parts of trees, shrubs, and some plants. As I did this over the years, a picture of this disease began to emerge. Some disease characteristics seemed to be common to all tree species, while others were specific to certain species. Collectively, all this information helped to characterize this disease.

Now, I realize that few people have the passion that I do in trying to definitively determine the identify of a strange and possibly new plant disease. Still, it is possible to use just a few diagnostic tools to greatly facilitate the identification effort. The next section describes the tools I used.

Unaided Eye White Canker Symptoms

After examining hundreds and hundreds of plants, shrubs, and trees over a period of years, a common set of white canker symptoms has emerged that are visible to the unaided eye (microscopic symptoms are discussed later). Many of these symptoms are common to other diseases, too, just like a person having a fever doesn't point to any particular disease, just that some disease is present. But, taken as a whole, the set of symptoms described below is a strong indicator of white canker.

The overall appearance of a tree will often change, depending upon the tree species. For example, in blue spruce trees, branch death will proceed from bottom to top, and from inside out. Branches will sag and experience more tip growth. Red oak trees will experience branch death from the bottom to top over the years. On many trees, the branches will get "leggy", in that branches will grow longer but only have leaves near the tip.

Some trees that are known for their colorful bark (e.g., Japanese Stewartia and River Birch, will lose much of that interesting color, turning more drab.

In mid to late summer, the stress on a tree can become so great that the tree will shed its leaves. Birch trees often do this. White pine trees are doing it now.

When a tree is infected with white canker, the fungus growing inside it will weaken the wood. Some trees, such as Colorado blue spruce, normally hold their branches very horizontal, making for an attractive appearance. But when infected with white canker, their branches tend to droop, and "pancake" one another, as shown in the photo on the left. When this happens, some homeowners will simply prune out the lower branches. But this does little good, as the branches above the pruning then also droop further down.

The leaves will both feel and appear to be very dry, as if starving for water. They may seem limp, and lose their sheen. (For comparison, a healthy leaf is usually very flat and has a shiny surface.) The outer edges will be especially dry, and may even die. As this happens, the leaves will curl, as if trying to protect themselves from water loss. If the tree is already badly infected in spring, the new leaves will appear withered or badly wrinkled.

The cause of this deformation is an abundance of cankerous material growing within the leaf, which both physically deforms the leaf and robs it of its nutrients. This weakening may have secondary consequences: other diseases, such as anthracnose (black irregular blotches) may move in, and insect damage may increase. Also, the stress of exhaust from cars will cause those leaves close to a road to die first.

The bark on the tree's trunk will appear stretched, as if the tree is growing faster than the bark can accommodate. On some trees this will result in one or more or deep vertical fissures (e.g., oak, maple, and pine). In other trees, the bark will uniformly expand, resulting in numerous vertical fissures. The bottom (deepest part) of these fissures will usually have the color of peanut butter (a mixture of normal dark brown bark and white canker material). Starved of supporting nutrients, the bark will also appear unhealthy, as if it is rotting on the tree. In fact, some of the surface bark may often spall off. This "sick" bark will often support a much larger population of green-gray lichen than normal, causing the tree to appear as if someone had painted one side of it with green paint.

Up to about June 5th, the bark on small branches will have tan patches on them, as if they had been spray painted (see the picture at right). These patches primarily appear on the bottom of the branch, near branch junctions, and they are actually composed of numerous fruiting bodies. Since these fruiting bodies do their work in early June, this evidence mostly disappears after that. These patches, while hard to locate, are a sure indication of a white canker infection.

As the disease increases in severity, holes may open in the bark and the tree may "bleed" sap from them. This sap is odorless and light brown in color. Pinkish-white lichen will often grow near these bleeding areas.

The fruit or flowers, if normally present, will be small or won't be there at all, since the tree doesn't have the energy to support them.

Overall, the tree will slowly decline and die, usually branch by branch. But it's very unusual for white canker to kill a tree in a year or two. Since white canker is a fungal disease, if the fungus kills its host, the fungus kills itself! Instead, tree death occurs over a period of several years, e.g., 3 to 12. Fortunately, that leaves time to implement chemical control (covered later).

Microscopic examination (400x microscope) of the leaves and twigs of an infected plant will often show fruiting bodies that resemble pollen - especially before about mid-June (here in New England). These fruiting bodies are about 50 micrometers in diameter, are somewhat tan in overall color, and have the consistency of flour in your hand. See the picture on the right. Fruiting body density usually correlates to both infection severity and location, with the highest density at nodes (branch or leaf junctions). But they can appear between nodes and on leaves too. These tan patches of fruiting bodies are definitive of a white canker infection, as we'll see later. No other unaided eye evidence is definitive.

But like mushrooms, these fruiting bodies are only the surface view of the infection raging under the bark and within the leaves. Specifically, a fungal-like infection is causing a white-colored canker material to consume the green nutrient-laden phloem tissue which the tree needs to survive. As the canker invades the phloem, it initially expands (pushing the bark out), sending out ribbon-like threads (hypha) to infect other areas of plant tissue. As the infection deepens, the green phloem is killed and significantly shrinks, often leaving voids.

One potential way to highlight diseased wood is to use wood staining chemicals. Hoping that staining would make the white canker stand out, I bought some basic staining chemicals and gave them a try. Specifically, I tried Toluidine Blue (1%), Methylene Blue Chloride, Iodine Potassium Iodide, and Carbol Fuchsin. A drop or two of each was applied to several areas of a 2-inch diameter slice of a chokecherry tree branch that had obvious signs of white canker infection. While the wood did soak up the chemical, the result was simply two dark blue spots, a dark red spot, and a light yellow spot. Microscopic examination of these spots showed that the white canker substances took up the stain the same as the adjacent wood. So, unfortunately, my staining attempts turned out to be a dead end.

My Diagnostic Tools

As I mentioned earlier, when you examine diseased plant tissue, it's very helpful to compare it to plant tissue having a known disease. While that sounds obvious, it turned out to be very frustrating to me. The reason is that different plant diseases result in similar appearances, e.g., "spotty wilted leaves". It's kind of like going to a doctor and saying "Doc, I have a headache and feel achy all over. What's wrong with me?". And when you persist in trying to understand the details a particular plant disease exhibits, the text will often say something to the effect that it's very difficult to diagnose, and you should see an expert!

USB Microscope
A major shortcoming, in my opinion, of plant disease information out there is that so much of it seems really dated/old. Specifically, many photos are in black and white. And many are in low resolution. And for some reason, the vast majority of diagnostic photos are "eyeball" photos - a hard copy of a view as seen by an unaided eye. But there is a far greater amount of diagnostic information that is only available when looking through a microscope. The average person probably assumes a microscope is costly and bulky, and that you have to look through an eyepiece to see plant tissue samples carefully prepared and placed in glass plates. But that's not necessary at all. Technology has advanced to where you can get an inexpensive hand-held microscope that plugs into your computer's USB port and displays beautiful close-ups on your computer's display screen! And you can capture the displayed image into a standard JPG picture file for archival and later study. These days, virtually everybody has a computer with a USB port.

Consequently, in addition to my computer, my main diagnostic tool is a USB microscope. While there are many manufacturers of these microscopes out there, I happened to choose a Celestron model 44300 that had a maximum magnification of 400 power (400x). I would have gotten one with even higher power, but 400x is about as powerful as they come. At the time I bought this one, the cost was about $100, but the cost has come down since then.

Flatbed scanner
In addition, I supplement this with a USB flatbed scanner that can scan at up to 3200 dpi. I use it for scanning small branches, sets of leaves, and individual leaves. I find that a resolution of 2400 dpi works out well. One nice feature of these scanners is that the depth-of-focus isn't terribly critical, meaning that something like a leaf doesn't have to lie perfectly flat in order to obtain a clear image. A USB microscope is far more critical regarding focusing.

Photo editing software
While the microscope and scanner both generate picture files, there is often a need to crop them, rotate them, adjust them for lighting, etc. For that I use Adobe Photoshop Elements, since it's popular, relatively inexpensive, and has a wealth of photo editing tools that can be used for a variety of purposes. But most any good photo editing software will do. In addition, I also use Photoshop Elements to embed documentation on the photo within the photo's EXIF data block (File|File Info...|Description). Some information items I include are the name of the plant in the photo, the date it was taken, where it came from, the size of the specimen sampled, and particular areas of the photo to note for diagnostic purposes.

One handy feature of Photoshop Elements is the ability to easily interface with the USB microscope and scanner.

Photomicrograph: A micrograph, or photomicrograph, is a photograph or digital image taken through a microscope or similar device to show a magnified image of an item. For more information, see the Wikipedia definition.

One limitation of Photoshop Elements is that it can only handle a maximum of 200 captured photomicrographs.

Cutting tool
Whether looking inside a leaf or twig, it's necessary to make a cut. For rough twig cuts, I use a generic branch pruning tool. Unfortunately, pruners such as this often crush a twig and make a rough cross-section. Consequently, my rough cuts are done with pruners, while my analysis cross-sections are done with a single-edge razor. While a razor gives a nice clean cut, it becomes really difficult to cut twigs greater than 1/8" in diameter.

To do a cross-section of a leaf, simply clamp the leaf in the vise, then use the single-edge razor to cut the leaf along the top of the vise jaws.

Hobby vise
Another simple, but crucial, tool is a hobby vise. I use its jaws to grip samples. Here are some tips to aid in subsequent picture taking by a USB microscope:

  • The jaws should have a flat top so that the microscope's round plastic hood can easily rest vertically on the jaws. This is important since the depth-of-focus at 400x is extremely critical, and you want to have to refocus as little as possible in order to save time.
  • Some vise jaws are made of a relatively bright material (e.g., Teflon), and it will somewhat wash out the dark green of the sample. I get around this by placing black electrical tape over the vice jaws.
  • Use a needle-nose pliers to facilitate placing the sample twig in the vise jaws.
  • Let the top of the twig sample extend slightly above the vise jaws. That way the jaws will be out of focus and won't be in the photos. Also, vise pressure will be less likely to crack the woody tissue.
  • Try to mount the twig sample so that the cut surface of the cross-section is parallel to the vise jaws. This ensures that the microscope focus doesn't have to be continually adjusted as you pan around the twig cross-section.
  • One of my hands holds the microscope vertical on the vise jaws while the other hand clicks the mouse to snap the photogmicrographs.

Photo stitching software
Later in my diagnostic work I added what I also consider to be a very important tool - the free Microsoft program ICE (Image Composite Editor). I've found that while 400x photomicrographs can be very informative, they capture only a very small piece of the target. Yet it's often the "big picture" that gives one good insight as to what's going on.

Adobe Photoshop Elements also has the ability to stitch images together. On numerous occasions I've tried to make use of it. But it is slow and can only handle a limited number of photos (maybe less than a dozen). Sometimes it seems to hang, and sometimes it generates a distorted result. This was with version 11. Possibly newer versions have improved. On the other hand, ICE is relatively fast, never hangs, never distorts the result, and can stitch over a hundred photos. My record so far is 210 photos stitched with ICE, although it takes about 15 minutes of processing to do so.

Putting it all together
Here's how I make use of all the information above to make a twig cross-section:

  1. Snip a candidate branch from a tree (or stem from a plant).
  2. Snip off a sample that is about an inch long and about 1/8" in diameter.
  3. Use a single-edge razor to make a clean cut at one end.
  4. Mount the sample with the cut edge up and parallel to the vise jaws.
  5. Bring up Photoshop Elements, and use (File|Import ...) to enable the USB microscope.
  6. As my left hand moves the microscope barrel over the cross-section, my right hand left-clicks the mouse to capture successive, overlapping photomicrographs (overlap by about 50%).
  7. Repeat until the entire cross-section has been photographed (usually 30 to 200 photomicrographs). Try to not rotate the microscope during this process, or the subsequent stitching operation may fail.
  8. Save each image in a folder of your choice (the individual file names are irrelevant, so you can use the default).
  9. Call up the ICE program, select (File|New Panorama...) and select all files in that folder of photomicrographs that you created, and ICE will begin the stitching process.
  10. When the stitching operation is complete, save the stitched picture in the same folder, giving it a meaningful name.
  11. Load that stitched picture into Photoshop Elements.
  12. Correct for color accuracy (I use (Enhance|Auto Levels)), and crop and rotate if necessary.
  13. Include descriptive information regarding the stitched picture in the EXIF data.
  14. Save the updated picture and exit.

Typically, a 1/8" twig will generate a JPG file having a resolution of about 2500 x 2500 pixels. Unfortunately, this entire process from start to end can take up to 45 minutes. On the other hand, the resultant picture is very insightful as to the internal health of the plant!

Microscope Aided White Canker Symptoms

As discussed above, computer-connected USB microscopes are easy to use and have come down in price dramatically over the years, to the point where they are available for well under $100. So if one is serious about identifying white canker themselves, it is probably a good investment. The reason for this is that white canker can be positively identified with such a microscope. In particular, you can see the white canker's fruiting bodies and its hypha within the diseased plant tissue. You can also see the massive internal destruction of plant tissue it causes, making it obvious as to why the plant is dying.

As bad as white canker is, the good diagnostic news is that it lives almost everywhere on the plant! You just have to know how to recognize it, much as you learn how to identify different flowers or mushrooms (fungi) by their color and shape. Specifically, white canker lives on the outside (bark) of plants and also inside the tissue of plants, where it does most of its harm. And by "inside", I mean in the phloem, the xylem, and the cortex. A 400x microscope can show you this.

Fruiting bodies on the bark
Previously I mentioned that the one surefire way of diagnosing a white canker infection in a plant was to look for tan patches at plant nodes/junctions, especially the side facing down. Since white canker is a fungus, it has to have a means to reproduce. Hence, there has to be some sort of fruiting body on the surface of an infected plant that spreads reproducing spores that will blow away and infect other plants. A mushroom is the fruiting body of a fungus that also spreads spores that are dispersed by wind, rain, birds, people, cars, etc..

Spore: a reproductive structure that is adapted for dispersal and surviving for extended periods of time in unfavorable conditions. Spores form part of the life cycle of many plants, algae, fungi and some protozoans. A chief difference between spores and seeds as dispersal units is that spores have very little stored food resources compared with seeds.

By chance, I happened to stumble upon these fruiting bodies while I was pruning an overgrown chokecherry tree. The distinctive fissures in its bark and prior leaf damage strongly indicated that this tree was infected with white canker. My prior observations seemed to indicate that white canker attacks the bark of a tree. On a hunch, I cut off a diseased-looking branch, placed it on my computer's flatbed scanner and scanned it at a high resolution (2400 dpi). Parts of the branch looked like they were covered with sawdust, or a salt-like substance. Wondering if that was significant, I checked out branches from other diseased trees. Sure enough - the same tan flecks were present. Frustrated in not being able to get a closer view, I bought a 400 power hand-held digital microscope.

A microscopic view showed that this "sawdust" turned out to be a field of snow-white "pac-men-like" objects and yellow blobs. Not only that, but there were also translucent ribbons that looped from location to location, sometimes forming spider-like objects. Wow - a whole new world!

But was this new set of objects meaningful in diagnosis? To find out, I looked for these disease objects on infected and non-infected wood. The correlation was there - the worse the infection, the more of these objects I found. But - you often had to know exactly where to look for them, which may be why others haven't noticed them. As my investigation progressed, I became more and more convinced that I was now seeing the fungus that was causing the disease.

Then I used the microscope to examine a cross-section of some small branches. Expecting to find healthy phloem tissue and bark, I instead found a mess of transparent ribbon-like objects, white waxy blobs, and what looked similar to the fruiting bodies I had seen on the bark. Once again, the more disease the tree had, the more the bark and phloem was eaten up by this other stuff. And once again, I saw this same infection pattern on the cross-sections of other diseased trees and shrubs.

It soon became apparent that these fruiting bodies and characteristic white cankerous growths were highly significant diagnostic markers for this fungal disease - which is why I call the disease white canker.

Fruiting bodies on the leaves
With a strong white canker infection, fruiting bodies can even be seen on the leaves, both on the upper surface and on the lower surface. If present, they generally tend to congregate near the veins (especially the midrib), where their food supply is relatively plentiful. They are even more likely to occur where the leaf stem attaches to the host plant stem.

The white canker fruiting body
These white canker fruiting bodies have the following characteristics:

  • Shape: Mature fruiting bodies are bi-lobed, milk-white, with an upward-facing cleft, making them resemble a 3-D, fat, and upward facing pac-man. Sometimes their color and shape reminds one of teeth. The pac-man "mouth" can be open or closed. Young, or immature, fruiting bodies appear to be smaller, more globular, and yellow or yellow-green in color.

  • Size: An individual mature fruiting body appears to be about 50 millionths of an meter (50μm) in diameter, or about one-tenth the width of the average human hair. That's pretty tiny, which is why you need a microscope to see them! The young fruiting bodies are even smaller - often about one-fourth the size of a mature fruiting body. A 10x eyepiece can be used to distinguish the individual fruiting bodies, but the distinctive shape is only apparent when viewing them at 400x. A large population of fruiting bodies will appear as a tan-colored area to the naked eye, and may be quickly passed off as a dirty smudge.

  • Location: The vast majority of fruiting bodies are located on the underside of branch and leaf junctions, about 1 to 5 feet from a branch tip. This wood is generally one to three years old. Fruiting body density can be so high that it totally covers the wood. Unless the plant is heavily diseased, you won't find the fruiting bodies anywhere else. So, unless you know where to look, and what to look for, you probably won't see them. Furthermore, when to look is also important. They are abundant just before they release their spores (early June). Far fewer are seen after that.

  • Fruiting body hardiness: These fruiting bodies appear to be very robust. I've tried rubbing my fingernail across them with no visual effect. So don't worry that handling the branch will rub the fruiting bodies off. I've found these fruiting bodies on live branches, half-dead branches, and fully dead branches that have been on the tree for over a year. Because of this, you needn't worry if you collect a branch sample but don't get around to checking it for these fruiting bodies for some time. The leaves may wither away, but the fruiting bodies will remain.

  • Fruiting body host preference: All the above fruiting body observations seem to apply no matter what the shrub or tree. Almost all trees and shrubs seem to have the same fruiting bodies in the same places. So far, the only difference I've noticed is that while all leaves eventually pucker or deform under the influence of this disease, viburnum bush leaves always seem to appear healthy. Similarly, while the bark of all trees is affected, the bark of a Ginkgo tree is not (but see the detailed photos later on).

  • Spore geographical distribution: I've seen the general leaf and bark symptoms of this disease here in Nashua, New Hampshire, in the northern part of our state, in Portland, Maine, in the Denver, Colorado area, in Milwaukee, Wisconsin, and down in Florida, so I suspect this disease is widespread. But I've only microscopically examined a northern Michigan white pine for these fruiting bodies - and they were present.

Plant pollen?
At this point I should mention a highly contentious issue. When a local state plant pathologist briefly reviewed my photos of these white canker fruiting bodies, she suggested they were just "probably pollen". In particular, white canker fruiting bodies are shed at about the same time of year that white pine pollen is shed. So I looked for pictures of white pine pollen on the Internet, and also examined containers of pollen from white pine trees. To my great surprise, they were virtually identical! Of course, that caused me to then question all of the white canker fruiting body analysis I had done. In fact, I spent many, many hours pondering this paradox.

But after studying thousands of microscopic photos of hundreds of plants, shrubs, and trees, I came to the conclusion that white pine pollen and white canker fruiting bodies actually DO look the same! Why, I don't know, but I suspect that white canker is a pathogen that somehow copied the genes for constructing white pine pollen. This issue will probably only be fully resolved in the future when a DNA analysis of white canker is performed.

Very strong evidence that these fruiting bodies I found are indeed produced by white canker will be shown in later photos within this document. (Here again, I like Dr. Shigo's advice regarding conflicts: if people say one thing and trees say another, follow the trees.)

Hypha: A hypha is a long, branching filamentous structure of a fungus.

Icicle-like ribbons among the fruiting bodies
While white canker fruiting bodies may appear to be "just sitting" on the surface of a plant (usually on the bark), each is actually attached to a supporting hypha. Multiple hypha are referred to as hyphae (HY-fee). When hyphae branch from a single location (giving a spider-like appearance), the structure is called a mycelium.

A white canker hypha is a thin, translucent, ribbon-like object, with a thickness of about 15 micrometers and a width of several times that size. Their slightly bumpy appearance makes them look like icicles. While hyphae are usually embedded within the plant tissue, and thus not seen, occasionally they will appear on the surface bark along with the fruiting bodies. In that case the free end will often be pointed. Sometimes hypha will have several twists, enhancing their ribbon-like appearance.

When a twig containing white canker is cut for the purpose of making a cross-section, the cutting device will sometimes drag some of these hyphae out across the surface of the cross-section, which then makes them visible with a microscope.

Cortex: Inner bark tissue that lies between the outer bark and the phloem. Within branches, it is often dark green in color.

Fungal Growth Within the Plant
All the plant external diagnostic observations presented so far don't really give us any insight as to why the plant is in decline or dying. During my microscopic investigations, I've found that by looking inside the plant, it is possible to guage the extent of white canker infestation, and hence the health of the plant. To do this, you certainly don't need to chop down a plant! In fact, only a leaf or twig is needed. And if you use a twig, I've found that a twig having a diameter of about 1/8" or less will work just fine. Larger diameter twigs yield little additional information, and are more difficult to process, e.g., cut cleanly with a razor.

Leaf cross-sections
Surprisingly, a leaf cross-section is useful in diagnosing white canker because the white canker fungus is also present within the leaf! To see this fungal growth, clamp the leaf vertically (stem down) in a hobbiest vise. I generally prefer to have about 2/3 of the leave sticking above the vise jaws. Then, using a single edge razor, cut through the leaf parallel to the vise jaws, so that no part of the leaf extends above the vise jaws. Now, rest the hand-held USB microscope on top of the vise jaws such that it is looking down into the cross-section of the leaf.

If the leaf is moderately to strongly infested with white canker fungus, you will see white strands and white blobs among the green leaf tissue. While this white canker may appear throughout the entire cross-section, I've found that it tends to concentrate around th leaf midrib. In fact, for light to moderate infections, this midrib area is the only area that may show the white canker. Consequently, when I examine the cross-section of a leaf for white canker, I now only examine the midrib area. Unfortunately, I've found that while the midrib area may show little infection, there definitely is a white canker infection taking place, as evidenced by twig cross-sections.

Twig cross-sections
Up to this point I've discussed various diagnostic tests one can use to diagnose white canker, and I've used them all. None of them, however, gives a high reliability test for white canker. But based upon many hundreds of tests, I've found that microscopic examination of twig cross-sections can almost always detect the presence of white canker, along with how pervasive it is. The reason for this is that the white canker fungus (hypha) is very light gray (almost white), and most healthy internal plant tissue is green. And while some internal plant tissue (the xylem and pith) may be white, its normal patterning will be altered by being intermingled with blobs of white canker coloration. Therefore, no diagnostic staining is necessary.

Photomicrograph: A photograph taken with the help of a microscope.

While experience has shown that a 1/8" diameter twig is ideal for doing a twig cross-section analysis, a 400x view of a twig this size only shows a very small portion of it. In fact, it's very difficult to infer much diagnostic information at all with such a small view. Hence, as covered previously, dozens of 400x photos of this twig cross-section are made, and they are all stitched together into one big picture.

As interesting and potentially informative as these twig cross-section pictures are, they are only useful if one knows what to look for in them. In other words, we need to first know what what a normal cross-section looks like. Unfortunately, that's not easy. In an extensive search of the Internet for cross-sections of twigs and stems, I found NO cross-sections of real plants in natural color! There are plenty of black-and-white sketches, and some colorized drawings. Some were labeled, but even if they were, labeling seemed inconsistent.

Consequently, I decided to show a few example cross-sections from among the many cross-sections I've made. Since I was so far unable to find a shrub or tree that was totally free of white canker, I selected some trees that so far haven't shown a severe white canker infection.

Figure 1 shows a cross-section of a linden tree twig. I've followed this linden tree over about 8 years. Each year it visibly suffers from white canker late in the summer by having its leaves turn yellow and drop. But the tree seems to bounce back every year.

Figure 2 shows a cross-section of a blue spruce twig. I've also followed this blue spruce tree over about 8 years. Each year it sheds more needles. The lower 5 feet became totally bare, so the homeowner cut off all the dead branches. But the disease continues to advance upward. Like other blue spruce trees in the area, this one will eventually die.

Figure 1 - Linden tree twig cross-section Figure 2 - Blue spruce tree twig cross-section

Next, note the colored arrows pointing to various internal structures within each cross-section. It may help to click on each picture to zoom in so the detail is more clearly shown. The colored arrows show the following:

  • Brown arrow: Outer bark
  • Red arrow: Phloem, which transports nutrients up and down the plant
  • Blue arrow: Xylem, which transports water and minerals up to the plant
  • Cyan arrow: Pith - which stores and transports nutrients throughout the plant
  • Orange arrow: White canker (fungal hyphae)
  • Violet arrow: Figure 1: Pith ray, Figure 2: Vacuoles, which transport sap

There are several things to note in these two figures:

  • The bark should be pure brown throughout. Instead, parts of it are white, meaning those parts are filled with white canker.
  • Underneath the bark is a green band of phloem. It should be a dark, rich green. Instead, it is whitish, with scattered dense areas of white canker.
  • In Figure 1, the xylem lies under the phloem. It generally is somewhat green in color. However, here it is white and yellow, meaning it is infiltrated with white canker.
  • The center pith should be a frothy white, like soap suds. Part of the pith in Figure 1 is like that, but part is solid white canker. In figure 2, much of the pith is filled with white canker.
  • As Figure 1 shows, white canker can be diffuse, form in bands, or form in fuzzy clumps.

Treating White Canker

Fungicide: Any agent that destroys or prevents the growth of fungi.
Finding an Effective Chemical Treatment
After seemingly getting nowhere while trying to determine the common cause of so many of my shrubs and trees dying or in declining health, I did a lot of searching the Internet for chemicals that might be of help. After finding a few candidates, I called a local tree/shrub/plant care company and asked them to spray my yard - something I'd never done before since I'd never had a yard disease this severe.

Based upon my Internet research, I specifically asked them to also include a fungicide called Mancozeb. They did this, although they call it Protect instead. Their Protect fungicide was used in addition to their fungicide called Eagle.

For the first 10 days there was no effect. But then it was like a miracle had happened. Every single diseased plant, without exception, perked up and the foliage turned a deep healthy green! I could hardly believe my eyes. It was like day and night.

But… was the improvement due to Eagle or Protect? Both were fungicides. The service's agent thought it was Eagle, since Eagle was systemic (works within the plant, rather than as a disease barrier). So, as a further experiment, in the next spraying a month later, I had them exclude the Protect. The disease got progressively worse again. When it was time for my third spraying in mid-August, leaves were falling from my (and others) birch trees like rain.

For the third spraying, I had the agent exclude Eagle, and just use Protect. About a week later the improvement repeated itself - the birch leaves stopped dropping and stayed green. The other diseased plants also improved. That was the proof I needed - fungicide Protect/Mancozeb was definitely effective in holding back this disease. But it didn't cure it. A treatment is needed every 3 or 4 weeks.

I continued working with the tree care agent for the following year, trying out several more fungicides to see how they worked. Most were effective, one wasn't, as summarized in the following table:

Chemical Name Product Name Effective? Type
Mancozeb Protect T/O Yes Contact
Propiconazole Spectator/Infuse Yes Systemic
Thiophanate-methyl Cleary 3336 Yes Systemic
Myclobutanil Eagle No Systemic

The fact that several fungicides were so effective seems to imply that this disease is a fungus or fungus-like (e.g., Phytophthora) pathogen. It also argues against the suggestions that the cause of this decline was: cold weather, hot weather, not enough water, too much water, insects, too acidic a soil, lack of fertilizer, too much fertilizer, or physical damage. Yes, incredibly, all these things were suggested as the cause of the tree and shrub health problems on our property!

There is no Permanent Cure
Once you determine that the cause of your tree and shrub decline is due to white canker, you will no doubt want to get rid of it permanently. But, like so many other fungal diseases, there is no permanent cure. This pathogen is, unfortunately, here to stay. The only thing you can do is to control it, keeping it at such a low level that it's no longer bothersome. So then the issue becomes whether or not to treat your plants for this disease or not. The easy thing is to do nothing. But the consequence of that is that your plants will continue to do poorly, and many will eventually die. If you do decide to do something, the issue then becomes who applies the controlling chemical.

Who Does the Treatment?
The first thing to keep in mind is that, in order to keep white canker in check, the fungicide selected must be applied every 3 or 4 weeks, since the effectiveness begins to wear off after that period of time. The next decision you have to make is who is to apply the fungicide - either you apply it yourself, or you hire a commercial lawn, tree, and shrub care company. I've tried both approaches. Each has their advantages and disadvantages.

Hiring a commercial firm to apply fungicides has both advantages and disadvantages:


  • Easiest way to go - they do all the work
  • Most expensive, costing anywhere from $50 to $100 per treatment, and you may need four or five treatments for the season
  • Treatment usually includes a mix of fungicides and insecticides, so you are protected against both diseases and insects


  • The company may not be willing to spray the particular fungicide needed to control white canker
  • The company may be backed up in work, and may not be able to spray the fungicide for 8 weeks or so
  • The company may not spray an effective fungicide because they want to rotate fungicides in order to reduce the possibility of fungicide resistance over time
  • They may not spray your trees, shrubs, and flowers out of concern harm to bees
  • Some chemicals may be harmful to miscellaneous wildlife that come into contact with the leaves or branches of the sprayed plants, trees, and shrubs
  • They spray your property on their schedule, not yours

Applying the fungicide yourself also has both advantages and disadvantages:


  • Relatively low cost - you only need to obtain a sprayer (available from local home and garden centers for around $30), and the fungicide concentrate.
  • You can spray on your schedule
  • You can spray only those plants you choose to spray
  • You can avoid using an insecticide that might harm beneficial insects
  • You can select the dosage and application frequency
  • If you spray the ground, you don't have to worry about killing honeybees
  • If you want to do a good deed and spray your neighbor's tree or shrub, you can (I did that for my neighbor's rose bush to demonstrate the effectiveness of propiconazole.)


  • The effective concentrated fungicides are only available at some home and garden centers, but can be ordered on-line (e.g., Amazon). Some, like propiconazole, come in various concentrations (e.g., 2%, 14%)
  • Some fungicides can be dangerous if not handled properly (I've been cautious when working with them and so far haven't had any problem)
  • It can be time consuming, and a lot of work!

Regardless of who applies the fungicide, there is the issue of getting the fungicide into the plant. Commercial firms will generally spray the entire shrub or tree. But they have a problem spraying tall trees, as their pressureized spray equipment can only reach up to about 25 feet.

Personally, having everything sprayed with chemicals bothers me a bit. It bothers me even more when I do the spraying and the spray particles are close to me. I worry that a small gust of wind could blow them into my face. Fortunately, my extensive microscopic analysis of white canker shows that the fungal disease works its way up the plant through the roots and the xylem and pith of the plant. Because of that, I've experimented and found that you can spray a particular systemic fungicide, propiconazole, onto the ground around the plant. Rain or watering will wash it down to the roots which will then transport the fungicide throughout the plant. I prefer this method of application since the spray nozzle always stays inches from the ground, greatly reducing any drifting fungicide spray droplets.

drip zone: The drip zone is an imaginary circle on the ground under the tree that corresponds to the outer edge of the leaf canopy.
To expand upon this, when spraying propiconazole on the ground, I spray the ground under the "drip zone" of the plant until it appears totally wet. Having dry, bare ground or dry bark mulch helps to see where you've sprayed and where you haven't. I often then reapply several minutes later, so to achieve a "double" dose, since I've found through experience that this double dose is actually more beneficial than a single dose in combating white canker.

Here's a troubling experience I had with a lawn care company. I contracted with them to treat my shrubs and trees for the season. But they didn't do the first treatment until the last days of May. By that time, plant growth was well established, so the bad effects of white canker were already apparent. When they did come to apply their first treatment, they didn't spray some of the trees and shrubs that badly needed a treatment! The reason was their concern for the bees - they didn't want to harm them. I commend them for that concern, but on the other hand, my declining shrubs and trees continued their decline. My solution, was to spray the ground around these shrubs and trees myself. Since I never sprayed the leaves, flowers, and trunks, there was no issue with the bees, butterflies, etc. And I believe that a healthy plant is much better able to resist insect attack.

After your yard has been treated with one of these white canker fungicides, you should notice an improvement in plant health in about seven to ten days. But there are several issues to keep in mind. One is that the improvement seen depends upon the growth state of the plant. If a shrub or tree branch has no sign of life, it will NOT come back to life. If there are just a few sickly leaves on the tree or shrub, these leaves may or may not improve, since they are already formed. But any new leaves will be be well-formed and healthy. Since the white canker fungus kills the internal plant tissue, it will take time, sometimes months, for the plant to heal that internal tissue, much as it takes your body time to heal from a serious wound.

Leaf appearance is a good indicator of the health of a plant. Healthy leaves will generally be 1) very flat, 2) relatively large, 3) deep green in color, and 4) have a shiny surface. In fact, I often find healthy leaves like this so pleasing that I almost appreciate their appearance as much as the flowers on the plant!

Tracking Propiconazole Effectiveness over a Two-Month Period
While fungicide propiconazole definitely works as a control of white canker, some may question how long it takes for an effect to occur, how often the fungicide must be applied, what effect it has on the interior of a plant, what parts of the plant are most and least affected, etc. I know I had these questions in the back of my mind. Consequently, I undertook a controlled experiment where I sampled twigs from a single treated bush over a two-month period, every few days generating a microscopic view of a cross-section of a twig from the bush.

Visit this web page to view these progressive twig cross-sections, a discussion of them, and the interesting conclusions obtained.

Microscopic Insight
In the course of reviewing microscopic photos of white canker infection on hundreds of trees, bushes, and plants, I've gained a good understanding of what this disease looks like. Nature is a great teacher!

Visit this web page for a variety of photos that illustrate the diagnostic power of a hand-held microscope.

Propiconazole Safety
When applying a chemical treatment to your property, there is always a concern about safety. I'll briefly cover some of these issues here. For further details, see toxipedia article
Propiconazole, Syngenta Crop Protection article, Propiconazole. or World Health Organization article Propiconazole.

Propiconazole is a fungicide and antimicrobial that was first used in 1981 to protect grass grown for seed. The EPA expanded its permissible uses in 1987, 1993, and 1994 to include several food crops, but it is also used to protect ornamental plants. It may be sold under names Tilt, Alamo, Banner, Orbit, and Quilt. It is available as an emulsifiable concentrate, flowable concentrate, ready-to-use liquid, liquid soluble concentrate, wettable powder, and dust (#EPA). It targets fungi, bacteria, and viruses affecting plants. This broad targeting means that it may be used as a preservative to wood, adhesives, paints, coatings, leather, paper, textiles, and other industrial products.

While it is often used for garden plantings, it is registered for use on a variety of food crops, most notably banana, nuts, barley, corn, stone fruits, wheat, cereal grains, citrus, oats, rice, rye, sugarcane, and wheat. Nuts, rice, and wheat account for the heaviest usage. Field soil half-life ranges from 9 to 461 days.

In humans, propiconazole has a moderately low acute toxicity. Some humans exposed to formulated products containing propiconazole have shown local irritant reactions. Research has concluded that propiconazole is unlikely to pose a carcinogenic risk to humans. Human excretion occurs at 95% in 48 hours. Exposure potential is low under actual field use and poses minimal risk of adverse effects.

Propiconazole can be slightly to highly toxic to fish. It has a relatively low toxicity to birds, mammals, and bees, and other non-target organisms.

When to Apply the Fungicide
My testing has shown that a fungicide application yields the best results when a plant is actively growing. The difference is between "great" results an "virtually no" results. In general, apply in early spring (e.g., May 1st in New England) when trees, shrubs, and plants begin to grow. Good results will also be seen later on in those plants, such as roses, that continue to grow during the summer.

This early treatment is so important because when white canker gets established within plant tissue, it takes a lot of effort for a plant to get rid of it. And when the fungicide does kill the internal white canker, the plant has to grow internally to heal these internal wounds. This is less likely to happen later in the season when a plant prepares to go dormant for the upcoming fall and winter seasons.

Factors Making White Canker Worse

After I realized my garden plants were virtually all infected with a serious case of this white canker fungal disease, some of my first thoughts were "how did my garden come to get this disease" and "why does it seem to be affecting my garden more than the gardens of other people". These questions kept haunting me since I considered myself a fairly good gardener, fertilizing my grass and plants, mulching my plantings, and giving them sufficient water (although I'm not that big on watering, my wife is). So our plantings certainly don't suffer in the traditional sense.

Consequently, whenever I was away from home, I looked for evidence of white canker infections wherever I saw trees and shrubs growing. At first I saw no pattern. But after years of searching, some patterns began to emerge. Even when I started noticing these patterns, some didn't seem to make sense initially. But, gradually, the evidence built up to the point where I could make some general predictions about the factors affecting the severity of a white canker infection. I'll cover these factors in the following paragraphs.

Bark mulch - a blessing and curse!
Actually, my garden wasn't the only one in our neighborhood to contract white canker. My next-door neighbor got it about the same time I did. Also, she is a model gardener, in that she had lots of lovely plants, shrubs, and trees around her yard. So it seemed bewildering that both of our gardens should come down with white canker at about the same time. There were actually several findings leading to the solution of this mystery. The first clue was my microscope research that showed that white canker was a fungal disease that reproduced by fungal fruiting bodies in and on infected bark. Then I realized that about the time our gardens contracted white canker, we had shared a truckload of bark mulch. We had done this for several years in order to save money. And at about the same time I noticed that bark mulch didn't smell pleasantly aromatic anymore - it had a moldy smell instead. Plus, it seemed to rot away faster. The evidence thus strongly suggested that we acquired our severe white canker garden infections from loads of infected bark mulch.

Yet, emotionally, I found it somewhat hard to accept that the highly recommended gardening practice of mulching was in fact destroying my plantings rather than helping them! So I started looking around at other areas of plantings, trying to gather evidence one way or another. One area I often frequented was our local supermarket. Their parking lot had numerous islands that contained young flowering trees. As is often the case in these commercial plantings, they used bark mulch extensively. But as I monitored these plantings over the years, I noticed that their health invariably declined, e.g., branches gradually died off until the entire tree died. Then they would replace it with another tree. And that tree, too, would sicken and die over the span of several years. The evidence was just too strong to ignore - bark mulch, as attractive as it looked, had become a tree and shrub killer.

Water - another blessing and curse!
As I mentioned above, my wife is very good about watering our yard. Yet in spite of this diligent watering, our trees and shrubs still didn't look healthy. We used sprinklers. Our neighbor watered her gardens with an automatic underground watering system.

As I drove the highways, I kept searching for healthy and diseased trees. After years of this, I began to notice another pattern: trees that seemed in decline due to white canker were generally in lower elevation areas that had plenty of ground water. I seemed to see this correlation over and over again. At first, it didn't seem to make much sense. But after reading up on fungal diseases, the literature said that plant fungal diseases thrive in wet environments. Once again, another piece of the puzzle fit - our zealous watering was both helping our gardens and at the same time making our white canker infection worse!

Wind shear
While observing probable white canker infections in the countryside over the years, I noticed another strange correlation. It often seemed that in a group of trees, the trees on the end, or those experiencing the most wind shear would tend to have the most severe white canker infections. Like the other correlations I had noted, I at first dismissed this correlation as a fluke - just a chance happening. But, again, I saw this correlation so often that I figured there must be a reason for it.

At first I surmised that these trees were getting attacked by huge numbers of white canker spores being blown at them by the wind. But my microscope research showed that white canker chokes off the vascular system of the trees, effectively starving them of water. The drying effect of the wind pushes these infected trees over the edge, hastening their decline and death. A consequence of this reasoning is that dense areas of trees should show little decline due to white canker, and I've found that to be true.

So while wind doesn't cause white canker, it hastens the decline of trees and shrubs infected with white canker.

Vehicle exhaust
Another curious correlation I noted while driving around the countryside was that there seemed to be white canker decline on the branches of a tree next to a highway. Branches on the same tree facing away from the road, or higher up in the air, were in less decline. When I happened to mention this to a plant pathologist, she said it was known that road salt seepage would do this. Subsequently, I looked more carefully. One major highway I traveled was cut through about 10 feet of rock ledge. I noted that trees were in decline on top of this ledge. Yet it seemed very unlikely that road salt would climb this vertical ledge to the trees on top, whose roots clearly did not come in contact with the ground at the bottom of this ledge.

Then there was a road in our housing development. It had relatively light traffic, and I found no decline in the health of trees abutting the road, in spite of this road being occasionally treated with salt in the winter. But... there was an exception. Some of the overhanging branches of one particular tree were in decline or dead (the rest were in much better shape). It turned out that this tree was about 20 feet from a school intersection, and parents would often park their car there while awaiting the release of their children from the nearby school. Of course, when it was cool outside, they would let their car idle, causing car exhaust to drift up into the adjacent tree.

So once again, like with the wind, I don't think car exhaust has anything to do with causing white canker, but car exhaust is stressful to trees (as it is to us), and simply accelerates any decline due to a white canker infection.

Yes, there IS hope!
Given the above, it would be easy to be discouraged about gardening, since the two traditional good gardening practices, mulching and watering, now additionally can cause garden decline! But, as I've discussed earlier, there is a means of control, and that control involves the application of an effective fungicide, e.g., propiconazole, to all garden plants, especially if one is using bark mulch. The downside is that it does take time and/or money, although not an excessive amount.

Trees and shrubs in the country will have to go it alone, surviving as best they can. That means trees in wet areas will continue to die off. Susceptible trees will also continue to decline and die. In particular, before 2003, I recall that white birch trees were rather common. These days it's rare to see a healthy white birch.

When Will It Die?

When I see a tree that's infected with white canker and that is not on my own property, I often wonder how long it will be before it dies. That question probably runs through a lot of homeowner's minds too, if they own a tree infected with white canker.

A few years after I first encountered this disease, I thought the prognosis was grim for a tree infected with white canker. That was based upon my own experience with our Kwanzan cherry tree which became infected and then went into a steady decline until it finally died. At that point in time, one key indicator of this disease was vertical splits in the bark. But since that time, while the white canker infection continues, the bark splits seem to have diminished. At the same time, some trees that looked like they might die soon seemed to recover, only to show the same symptoms the next year. Yet other trees continued a slow, steady decline.

Consequently, predicting when a tree infected with white canker will die is much more complicated that I previously thought, as there seem to be a number of factors involved. I've covered those factors in detail within the preceding section, but in summary they are: the use of bark mulch, ground moisture, and wind. The presence of any of these three will hasten the decline and death of a tree infected with white canker. In addition, a possibly more important factor is the type of tree, bush, or plant. For example, while linden trees do get infected, they tend to recover each year. Colorado blue spruce and hemlock trees go into a slow and steady decline until they finally die due to needle loss, which may take 8 to 15 years. Furthermore, trees and shrubs seem to have an immune system which takes a year or so to activate. Like our own immune system, a tree/shrub can learn how to combat a pathogen, so that it is better able to fight it in succeeding years. That implies that one can't gage the eventual death of a tree or shrub by just following it for a year or two.

Still, it would be useful to have some means of measuring the health of a tree to see how close to death it was. It might seem obvious that one simply has to look at a tree to judge its health. Yet years of observation and experimentation have demonstrated that trees have a tremendous amount of resilience when it comes to handling disease or injury. For example, I had a mulberry tree I wanted to cut down. But before I did that, I progressively removed the inner and outer bark of the tree in an attempt to starve it. That's supposed to be a sure-fire way to kill a tree. Yet the tree didn't die until I removed 95% of the circumference of the tree! What this means is that a tree can be well on its way to death, but only give visible signs of death in the year or so before it dies. Before that, growth may simply stall.

Yet there is a better way to judge the health of a tree/shrub than by visual appearance, and that way is by its internal appearance. That can be done by cutting a twig from the tree and examining a cross-section of it. This lets us examine the phloem, xylem, and pith - those components of the tree that transport nutrients, water, and minerals throughout the tree, as well as store food. This transport system is called the vascular system, and is equivalent to our own blood circulatory system. If the vascular system is blocked, the tree can no longer live. White canker fungus kills a tree by totally infiltrating its vascular system. Yet, because of the built-in resilience of a tree, its vascular system can be 50% blocked, and the tree will still appear visually healthy on the outside!

Therefore, a good way to judge the health of a tree infected with white canker is to simply do a cross-section of a twig, and then see how much of the healthy deep green tissue has been replaced with white canker fungal hypha. For example, if there is very little green phloem left, the tree (or branch of it) is near death.

Figure 1 below illustrates this. This Colorado blue spruce twig should have a nice deep green phloem layer(the area immediately under the bark). Instead, this tissue is a "smoky" white with only traces of green. That smoky white is actually white canker fungus, which has almost totally taken over the nutrient transport system of the branch.

While the branch in Figure 1 still had needles and looked reasonably healthy, the cross-section shown in Figure 2 was from an immediately lower adjacent branch that had just died. As you can see, it has NO green phloem left. The only sign of life is a bit of sap exiting from a few of the vessels.

Figure 1 - Blue Spruce 2 twig cross-section Figure 2 - Dead Blue Spruce twig cross-section

Another tree that seems to go into a steady decline when infected with white canker is a hemlock. Figure 3 is a cross-section of a hemlock twig whose parent tree has been in steady decline for over six years. Each year a few more branches die, and the tree is now about 70% dead. Zoom in for a close look and you will see that here, too, the normally green phloem looks very damaged and there is hardly any green left. Once again, there is a lot of that white smoky canker present.

Figure 3 - Hemlock tree twig cross-section Figure 4 - Sugar maple tree twig cross-section

In contrast to this hemlock, Figure 4 shows a cross-section of a sugar maple that appeared to be in distress. While this neighborhood tree was dropping lots of leaves in mid to late August, it still had plenty of leaves left. And while the center pith was a normal frothy white, the immediately surrounding xylem was white with canker. As you can see, much of the normally green phloem was infiltrated by white canker. Most of the upper phloem was in especially bad shape. The decay was so bad that voids had formed in the phloem under the bark. In spite of all this, there apparently was enough green phloem left to support most of the leaves - once again, a tribute to the resilience of trees!

Diagnosis Example - Blue Spruce

One of the most common ornamental trees in use today is the blue spruce. Its conical shape, blue-green needles, dense foliage, and minimal care requirements make it a highly desirable tree for both homeowners and businesses.

Unfortunately, since about 2003, older blue spruce trees have started to go into a slow and steady decline. (Younger blue spruce trees resist this decline.) This decline was so slow that I'm guessing that few people noticed it. But if you look closely, you will notice the following disease characteristics:

  • The lower and inner parts of the tree show the most decline - a loss of needles.
  • Over the years, this needle loss works its way up the tree.
  • The branches on the lower half of the tree begin to "pancake", collapsing onto lower branches and leaving voids in the normally dense foliage.
  • The very symmetrical look of the tree is altered by occasional branches showing abnormal outward growth (i.e., sticking out), and then the tips curve upward.
  • Needles, especially on the smaller inner branches, turn brown and fall off. If there is snow on the ground, you can often see the surface covered with dead needles. If there is no snow, try shaking a branch - you will likely see needles fall like rain.
  • Needles on the branches about a foot from the tip begin to take on a slight yellow-brown tinge.
  • Where you could never see the trunk of the tree before, due to foliage density, the trunk now begins to be easily seen as bare branches become more common.
  • As these symptoms progress, the beautiful blue-green color of the branch tips often fades, and you find yourself wondering if this is really a "blue" spruce!
As you can see, the progression of this disease causes these blue spruce trees to lose all of the attributes that people love about this tree.

You may wonder how common these disease symptoms are. They are very common. Not only that, but most blue spruce trees around the country are currently experiencing these symptoms! Naturally, there is a huge demand for a diagnosis of this disease, along with a need for a way to control this disease. Of course, plant pathology experts have become involved in this major tree health problem. They have suggested that the cause of this disease is one or several of the known diseases of blue spruce trees. When pressed, however, they will admit that none of the known diseases adequately describe the known disease symptoms. Here is a brief rundown of their suggested diseases. My comments follow, based upon an analysis of several declining blue spruce trees in my neighborhood.

Cytospora canker (Leucostoma kunzei, formerly Cytospora kunzei)
(For more information, see Cytospora Canker at the Missouri botanical Garden website.)

Figure 1 - Cytospora canker on blue spruce

This is the most prevalent and destructive fungal disease of Colorado blue spruce and Norway spruce. It rarely affects trees less than 15 or 20 years old. It starts on the lowest branches of the tree, and, over a period of several years, progresses upward. At first, needles have a purplish hue, eventually turning brown and dropping, leaving dry, dead, brittle twigs and branches. A conspicuous white resin or "pitch" covers the cankered portion of the branch or trunk, sometimes flowing several feet down the trunk of the tree. Within the cankered area, black, pinhead-size fruiting structures (pycnidia) of the fungus can be seen with a microscope or hand lens and are a positive sign of the disease. Cytospora affects all needles from the tip of the branch to the base.

Comments: Good match with the first symptoms. But no white resin was found. More importantly, these black fruiting structures were not found, and all branch needles aren't affected - the branch tip needles are the last to die.

Phomopsis (Phomopsis occulta)
Phomopsis is a fungal pathogen normally only found on young trees in nurseries and on tree farms. The initial symptom of Phomopsis infection is a barely discernible chlorotic flecking of the needles. The discoloration is very subtle and may be difficult to distinguish from mite injury. Infection of the stems results in very small purplish lesions below the bark. Cankers expand under the bark. A resinous exudate is often present on the stem in the area of the canker. Cankers are easily seen on infected branches by shaving the bark away with a knife. Needles on infected branches may turn brown or purple, then drop. The most dramatic symptoms occur in spring as new shoots expand, then rapidly wilt and die. The wilted tips curl and often turn a characteristic pink before turning brown and shedding needles.

Strangely, there a very few Phomopsis diagnostic photos.

Figure 2 - Phomopsis canker on blue spruce

Comments: Good match with needles turning brown and dropping. No match on resinous exudate. No match on this being a young tree. But most importantly, since following these declining blue spruce trees for years, I've never seen new shoots expand, wilt, then die.

Rhizosphaera Needle Cast (Rhizosphaera kalkhoffii)
One source claims "This is the most common problem seen on blue spruce samples that are submitted to Plant Disease Clinics." As in the diseases above, this disease is usually first evident on lower branches and works upward gradually. Second-year needles turn a purple or brown color and eventually fall from the tree. After several successive years of needle loss, branches may die. In general, trees appear to die from the bottom upward. In some cases, however, infections start higher on the tree, giving the appearance of scattered dead areas.

The disease can be diagnosed by looking at the discolored needles with a magnifying glass or hand lens. Small black spots (fruiting structures of the fungus) appear in rows in the infected needles. The fungus is actually emerging from the stomata (natural pore-like openings) that occur in lines on all sides of a spruce needle. Green needles may show these small black fruiting structures. Healthy stomata appear white, so the rows of black stomata are a diagnostic feature of Rhizosphaera.

Figure 3 - Rhizosphaera needle cast on blue spruce

Comments: Excellent match on the first paragraph, which is probably why so many experts point to "needle cast" as the cause. But there is a total match failure on the key diagnostic evidence - the small black fruiting structures. They are simply absent.

Stigmina (Stigmina lautii)
This fungus is very similar to Rhizosphaera needle cast, but under very high magnification, the Stigmina spore-producing structures appear hairy or feathery, while those of Rhizosphaera are smooth and spherical.

Figure 4 - Stigmina on blue spruce

Comments: Once again, a mismatch on the small black fruiting structures. They are simply absent.

Diplodia (formerly Sphaeropsis)(Diplodia pinea)
Also called Diplodia Tip Blight. The most conspicuous symptom of diplodia blight is brown, stunted new shoots with short, brown needles. A few brown needles at the tip of the current season's growth are the first evidence of tip blight. With a hand lens you may see minute black fruiting bodies at the base of diseased needles, especially under the papery leaf sheath. These are sure signs of this disease. Future tree goowth is often reduced by damage to or death of terminal buds during infection. Apparently Colorado blue spruce trees aren't much affected by this disease.

Figure 5 - Diplodia

Comments: Once again, a mismatch, since this disease doesn't display stunted new shoots with short, brown needles.

Spruce Gall Aphids (Adelges cooleyi)
This insect causes the formation of conelike galls on developing twigs, which deform, stunt and usually girdle them. Colorado blue spruce trees seem to have a fair resistance to them.

Figure 6 - Spruce galls

Comments: No match, since no insects were seen, and neither were the galls.

Spider Mites
Spruce spider mites cause a webbing around the needles of spruce trees. You can see mites and eggs on the branches with a magnifying glass. They are small (about 1/50”), and with all eight legs stretched out would just cover the period at the end of this sentence. Newly hatched larvae are pinkish in color, but turn dark green or dark red after initial feeding.

Figure 7 - Spruce spider mites

Comments: No match, since no insects were seen, and neither was their webbing.

So What is it?
After reviewing the above known diseases of spruce trees and concluding that the symptoms don't match any known spruce disease, it becomes more likely that this is a new disease. This hypothesis is strengthened by the fact that this blue spruce disease is so widespread and so devastating. And it is relatively new, seeming to initially occur about 2003.

As you may note when reviewing the above listed fungal disease symptoms, this particular disease shares many of their symptoms, making diagnosis confusing. Therefore, what's needed is some new and unique diagnostic criteria for identifying this disease.

If it's a fungus, it must have fruiting bodies!
If this disease is a fungus, it must produce external fruiting bodies in order to reproduce. In turn, like a mushroom (which is a fungus), these fruiting bodies must produce spores which drift off to infect other plants. And since this disease seems to be very common, it probably produces a lot of fruiting bodies. In fact, this disease does produce a huge number of fruiting bodies in late May and early June (here in New Hampshire). But, there is a catch - unless they are very numerous, they are difficult to see with the unaided eye. They appear on the outside of the branch, several feet from the branch tip. They are small, yellow-light-gray in color, and almost look like dust. And to really confuse matters, they look almost exactly like white pine pollen! Hence, even if people do notice them, they are probably ignored!

If you don't have a microscope, but do have a scanner, you might be able to detect them. Figure 8 shows a blue spruce branch junction that is only about 8" from the tip of a branch. The first (left) picture was taken at 1200 dpi (dots per inch), the second at 1800 dpi, and the third at 2400 dpi. If you have good eyesight, you will notice that this junction is sprinkled with tan dots. Each dot is only a few pixels in size, so you might easily mistake them for plain, old dust. (Click on each picture to zoom in.) But a view with a microscope will show them to be typical white canker fruiting bodies.

Figure 8 - Blue spruce fruiting bodies at 1200, 1800, and 2400 dpi

If it's a bad fungus, it must seriously disrupt the tree!
Given the devastation this fungus produces on the outside, the internal workings of the tree must also be seriously disrupted. But, strangely, there doesn't seem to be any diagnostic information on blue spruce trees (or any shrubs or trees) with regard to how they look internally!

In frustration, I decided to "look into" a diseased blue spruce by cutting a 1/8" diameter branch with a single-edge razor, mount it in a vise, and examine it with a 400x hand-held microscope. I also did this with diseased blue spruce needles. The results were a real eye-opener. After having done the same thing while searching for white canker in other plants, I became fairly proficient at recognizing the symptoms of white canker. And these diseased blue spruces were simply loaded with white canker! But, like the fruiting bodies, if you don't know what you're looking for, you probably won't recognize the fungus.

Now that I've tried many diagnostic techniques on hundreds of diseased shrubs, trees, and plants in order to diagnose white canker, I can say that twig cross-sections are by far the most definitive test currently available for the identification of white canker. The other tests mainly lend supporting evidence.

Figure 9 - Blue Spruce 1 twig cross-section

What the White Canker Fungus Looks Like
As they say, "a picture is worth a thousand words". Figure 9 is a 1/8" cross-section of a twig from a badly diseased blue spruce that has all of the symptoms of white canker. It is a composite (panorama) picture made up of 47 individual photos (sometimes called photomicrographs) that were made using a 400x USB microscope. You've probably never seen a picture like this before. In spite of extensive searches of the Internet, I never have either. In order to more easily see the fungal infection details, be sure to click on this picture to expand it so that it almost fills your display screen. A second click on a location within that twig will expand it even further. Click on your browser's back button to return to this text.

As a reminder, you may recall that a normal, healthy twig will have a nice brown bark, a deep green phloem layer under it, a lighter-green xylem under the phloem, and probably a white pith in the center of the twig. It's important to note that this diagnostic twig was taken from a main branch that had dead twigs from the trunk up to this twig. In other words, this twig was probably going to die very soon. When you examine this cross-section carefully, it's no surprise that death is imminent.

First, check out the bark. The outer bark is almost totally overwhelmed by white fungal material just under it. In many areas, this outer bark has broken completely away from the orange-brown inner bark. The inner bark is also almost completely infiltrated with white canker. Still, all that tissue corruption wouldn't kill the branch. Next in should be the solid deep green phloem layer. This is the layer of tree tissue that transports nutrients throughout the tree. Except for a small area in the lower-left, the phloem has almost been totally consumed by white canker. Next in is the xylem, which is typically a light green. But it's almost pure white due to white canker infiltration. This xylem tissue, by the way, transports water and minerals throughout the tree. So the tree is starving for both nutrients and water.

Those "holes" you see in the green phloem are vacuoles that transport sap throughout the tree. When I cut this twig with a razor, these vacuoles oozed sap into the cut, which is why the phloem and xylem has a "wet" look. And those thread-like objects scattered about are the fungal hyphae that are growing out of the cankerous material within the twig.

Reviewing all this, we can say that the internal appearance of this twig looks as bad as the external appearance of this tree. About the only good thing you can say about this cross-section is that it sure is colorful!

Figure 10 - Dead Blue Spruce twig cross-section Figure 11 - Blue Spruce 2 twig cross-section

As I said above, this was the first live twig on the branch - prior ones closer to the trunk had already died. So, out of curiosity, I did a cross-section on the adjacent dead twig. Figure 10 shows the result. As you can see, all that's left is some dead tree tissue - most of the tree tissue has been replaced by white canker. Note that a bit of sap still oozed through a few vacuoles, in the branch's futile effort to stay alive.

Figure 11 shows another live branch on this tree. I cut this twig near a needle junction (on the left). This twig, too, oozed copious amounts of sap in a vain effort to stay alive. As you can see, there is virtually no green phloem left, having been almost totally replaced with white canker. Furthermore, those white rays radiating out from the center pith - they are rays of concentrated white canker too.

Figure 12 shows a 400x photo of a small section of bark. What you see is a field of white canker fruiting bodies intermingled with the hyphae supporting these fruiting bodies. I'm not sure, but I'm guessing that the yellowish stuff within the "jaws" of these fruiting bodies are the reproductive spores of the fruiting bodies. Unfortunately, my microscope isn't powerful enough to resolve them.

Figure 12 - Small section of Blue Spruce Bark Figure 13 - Blue spruce Needle:
Base, Middle, and Tip

Going further, I scanned a needle from this diseased blue spruce at 400x. There were so many photos that I had to stitch them together to make several panorama pictures. Figure 13 shows the base, middle, and tip of a needle. Note that the stomata are white, not black, so none of the common blue spruce diseases are present. As you can see, there is little indication of white canker in these external views of a needle.

Figure 14 - Cross-section of 3 diseased blue spruce needles

Figure 14 shows a composite 400x cross-section of a set of 3 needles from this diseased twig. While there is definitely white canker present (white blobs), it isn't nearly as obvious as the white canker in a twig cross-section.

There are three important points illustrated during this diagnosis:

  • While one can get a reasonable diagnosis of white canker from looking at specifics from the overall appearance of a diseased blue spruce tree, a diagnosis of much higher confidence can only be obtained by carefully examining a twig cross-section with a microscope at 400x. Only then can the presence and extent of a white canker infection be determined.

  • As the cross-section of a live branch shows, a tree branch's vascular system can be almost totally blocked, and the branch and its leaves/needles can still remain alive. It's a tribute to the resilience of trees in the face of serious disease.

  • This is a relatively new disease which probably made its first appearance around 2003.
Treatment Example - Blue Spruce

The prior section contains a fairly good discussion of how to determine if a particular blue spruce tree is infected with the white canker disease. Once a blue spruce is determined to have white canker, the question arises as to whether it is possible to do something about it. While I have successfully treated smaller trees and shrubs, I have no large blue spruce trees on my property, and hence no experience with treating them.

A friendly neighbor offers his trees for testing
However, I do have a nearby neighbor, Joe, who is not only a good friend, but who also happens to have two large blue spruce trees. These trees are shown in Figures 1 & 2. If you click on them you will see bare branches on their lower half (clicking again shows even more details). These are the trees that the diagnostic tests discussed in the preceding section indicated had a severe case of white canker infection. Joe kindly gave me permission to do a test treatment on his trees. Both of these trees have shown signs of decline for about 8 years. While they are "blue" spruce trees, I often wondered if they really were blue spruces, as there was almost no blue to them at all! That observation turned out to be critically important.

Figure 1 - Treated blue spruce Figure 2 - Untreated blue spruce

Treatment details
For comparison purposes, I decided to treat only one of his two trees. Both were of the same size, age, and condition - ideal for comparison. I did the treatment myself - no lawn care company was involved. My treatment was based upon what I had found to be successful in the past. Specifically, I created a propiconazole spray mix using about the dilution ratio suggested in the instructions, i.e., 1/2 ounce of 14.3% propiconazole concentrate per 2 gallons of water in a sprayer container. Because I had previously used up some of that 2 gallons, I was only able to apply 3/4 gallon to the ground under his tree. I began by attempting to wet all the ground under the drip zone of his tree. Fortunately, the ground was bare dirt, so it was easy to tell where I had applied the mix. If a large root was exposed, I sprayed that too. While I had hoped to cover the entire drip zone, I only managed to cover 2/3 of a pie-shaped area under the drip zone. I could have returned to finish the remaining 1/3, but decided to wait a week and check the results. If there was no improvement over this period, I was going to spray the remaining 1/3.

The next day there was a moderate rainstorm. I was happy with this since I figured that it would wash the propiconazole down into the roots, where it would then be transported throughout the tree, since propiconazole is "systemic".

4 days later
About 4 days later, as I walked by this tree, it seemed that there was "something" different about the tree, but the difference was so small that I wondered if it was my imagination.

One week later - babies!
Then, exactly one week after the rain, I again examined the tree, this time far more carefully. My first impression was that it was "bluer". More careful inspection showed why: numerous "baby" blue shoots were erupting throughout the branches on the tree! I was ecstatic! Further examination of these new shoots showed that they were all from 0.5 inch to 1.5 inches in length, with most being about 1 inch in length. So it appeared that these new shoots were growing at the rate of about 1 inch per week. Older, existing needles showed no change. And, interestingly, branches over the 1/3 of the drip zone that I didn't spray were also showing these new shoots! So it doesn't seem to be critical that the entire drip zone be uniformly covered.

Figure 3 shows these visual results (click it to get a close-up view). The pink arrows show these new baby shoots erupting from a larger stem. The orange arrow shows a new single shoot erupting from the tip of an existing shoot. The red arrows show three shoots erupting from the tip of an existing shoot. In particular, note that the older foliage was a less-healthy yellow-green color (the entire tree was of that color before the fungicide application), and the new baby shoots are an attractive white-blue-green color. Furthermore, the bark on these new shoots is a distinctive orange color.

Figure 3 - New blue growth

Of course, it's always good to insure objectivity. So after noting the new growth on this blue spruce, I went to the other untreated blue spruce and carefully examined it. As expected, it had absolutely no new growth.

An inch a day?
A day later the owner of those blue spruces and I examined the new growth. I took a few photos of the branches and trees to post here. But the next day, while examining these photos, I became unhappy with their quality, so I went back to get another set of photos. As before, I couldn't resist examining that beautiful new growth. But it now looked a bit different. Those 1" new branches were now 2" in length. I didn't believe it, so examined a few more new growths. Same thing. Still in disbelief, I examined dozens of these new growths. All of them now averaged 2" in length. The conclusion was inescapable - there had been 1 inch of growth in about a day or so! The message here seemed to be that the tree, while appearing to be in deep decline, was basically very healthy, but was brought to its knees by white canker. Suppressing the white canker caused the tree to burst out with new shoots.

No smooth transition?
There was another strange thing. I wanted to find the point on a growing branch where the old, plain green needles turned to a blue color. I figured it would be a smooth transition on the young bark and that only the needle color would change. In spite of examining dozens and dozens of these new shoots, I never did find a single smooth transition. Instead, all of the new shoots erupted from a junction point where the bark bulged a bit (see the orange arrow in figure 3). So the branch tips were apparently not growing at the time, and the fungicide treatment initiated a new growth spurt, one that typically happens in the spring.

Where did the blue go?
In order to get a nice close-up view of some of these new blue-green-white needles, I clipped off a branch tip that contained only this new colorful growth. But upon bringing it home and visually examining it within 15 minutes of cutting it, I found that the beautiful blue-green-white color had morphed into a plain green color! This coloration fade also occurred on a branch containing both old and new needles. I'm guessing that the white canker remaining in the new shoot's tissue, now lacking ongoing fungicide transport from the rest of the tree, suppressed the needle's generation of blue color. The evidence that a reservoir of white canker still remains after this fungicide treatment is shown later on in this section.

Nevertheless, I thought it worthwhile to document a close-up of a blue spruce twig that was created within the previous 10 days. Figure 4 shows a 600 dpi scan of one lying on my flatbed scanner. Since the needles keep the stem raised above the glass surface, parts of this twig are out of focus. Figure 5 shows the same twig, but with the white scanner cover pressing down on the twig, flattening it. You can now see more details, especially since the scanner resolution was upped to 1200 dpi. Yet, in spite of these great views - no blue! Furthermore, that new shoot white outline on the needles is also missing. On the other hand, the white stomata dots on the needles stand out starkly!

Figure 4 - New blue spruce twig
(600 dpi)
Figure 5 - New flattened
blue spruce twig (1200 dpi)

The inside appearance
While the tree was rebounding on the outside, I wondered what this new growth looked like on the inside. So I cut off a branch, brought it home, and made a cross-section of a twig that had grown since the fungicide application. Sure enough, as you can see, the interior is in much better shape than the diseased twig from before. The new bark is much better looking, the pith is nice and white, and the phloem has lots of healthy green tissue. It still has some swirls of white in it, so some white canker remains.

Figure 6 - Cross-section of a new blue spruce twig
after fungicide treatment

White Canker fruiting bodies - already!?
Figure 7 shows a high resolution scan of a blue spruce branch that had grown during the past 10 days. At first glance, although it appears very healthy, it doesn't look unusual at all. But click on the figure and examine the junction areas pointed to by the red arrows. To my utter surprise, these appeared very much like the white canker fruiting bodies I had seen in the past. Or, were they just dust? There was only one way to find out - break out the 400x USB microscope and check them out.

Figure 7 - New blue spruce branch cluster

The results are shown in Figures 8 and 9. Each photo in Figure 8 shows a massive field of white canker fruiting bodies. As is almost always the case, they tend to cluster at branch junctions near the tip of the branch. But the real surprise was that they were present at all! The reason is that these fruiting bodies tend to form and get released in early June, when white pine trees shed pollen. This particular branch was only about 10 days old, so these fruiting bodies must have formed within the past 10 days - not two months ago!

Figure 8 - New blue Spruce fruiting bodies on junctions

What this indicates is that 1) the fungicide didn't kill or disable all the white canker in the tree, and 2) white canker fruiting bodies form when the branch junctions form, not afterward. Both of these points were somewhat of a surprise to me, and, once again, are more proof that these objects aren't white pine pollen.

But that's not all. As Figure 9 shows, these white canker fruiting bodies can also appear at the base of blue spruce needles. Granted, there are few present there, but they DO appear there. I haven't found them at any other location along the length of the needle. And, since these needles have grown within the past 10 days, these fruiting bodies must also have formed within the past 10 days. White canker fruiting bodies really seem to prefer to grow at tissue junctions!

Figure 9 - New blue Spruce fruiting bodies on needles

Time will tell what further changes take place in this tree, but for now the exciting news is that a ground application of fungicide propiconazole will result in healthy and vigorous new growth on the diseased branches of blue spruce trees.

A Bush that had been "Dead" for Years Comes Back to Life!

By accident, I discovered another interesting fact about this disease. About 1998 I bought a small Rose of Sharon bush and planted it in a far corner of our yard. It grew well, getting bigger each year. Then, about 2003 it died over the winter, leaving a dead stick the following spring. I left it alone, hoping it would come back over the summer. It didn't.

The following spring I still left it alone, hoping against hope it would come back. It didn't.

The next year, fed up with my reluctance to discard a dead bush, my wife went to dispose of it by breaking it off at the base. But while doing so, she was astonished to find about 8 healthy shoots (one with a bud) coming from the root collar! In disbelief, I had to agree that this long "dead" plant was looking good! The picture to the right shows it.

Here's what I assume happened. White canker struck this Rose of Sharon and killed it above the root collar over the winter. But the roots were still alive. Each year it tried to grow, but the new small growth (which was covered by adjacent plants) was almost immediately killed by white canker. A subsequent spring fungicide spraying around 2006 killed off the white canker, permitting the new growth to finally thrive. It has been healthy and growing ever since. And now, in 2014, it is producing an abundance of blooms!

I see this as interesting evidence that this disease tends to kill or harm a plant above its root collar, but does not always kill the roots or root collar. That's of particular interest since Phytophthoras are known to attack the roots of a plant. So this is one piece of evidence that says this is not a Phytophthora disease.

Shed Fruiting Bodies

Back around June 9, 2008, I and others noticed what appeared to be a "fog" in the air during a hot and windy day. Someone else commented that when he looked at the trees, they appeared to be "smoking". The cause appeared to be a blizzard of yellow-green "pollen". Curious as to what this "pollen" looked like, I used a strip of cellophane tape to capture some and examine it under a 400x microscope. To my complete surprise, the "pollen grains" looked like the infectious fruiting bodies that I had been seeing on all my diseased branch samples!

After this "pollen blizzard" subsided, I walked the neighborhood and noticed that this yellow-green pollen was much denser on the street under some trees than under others. After looking at dozens of trees, I noticed a pattern: the dense pollen was only under some species of trees, and even then it was the densest under trees that seemed to be suffering from white canker (as determined by sparse or diseased foliage and/or sick-looking bark). In particular, these pollen producing trees were white pines, blue spruces, oaks, and maples. The most prolific pollen producers appeared to be Norway maples. But maybe, as some suggested, this was plain old maple pollen. Well, plants give off pollen to pollinate themselves, in order to make seeds that in turn can make a new plant. But as I looked down, I noticed maple seed pods on the ground, indicating that the maples had already gone through their normal reproduction phase. And, oaks had already shed their pollen producing stringy objects. This stuff they were now giving off was not normal pollen!

But while all this pollen appeared to be the same color, was it really the same? To find out, I used cellophane tape to gather a pollen sample from under each tree that had a significant amount of pollen under it. True, there would be tree to tree mixing of the pollen, but the majority should have been from the tree directly above it, since the pollen density tapered off as the distance from the tree increased.

White birch
Norway maple
Blue spruce 1
Pin oak
White pine
Blue spruce 2
Red maple
Silver maple

A 400x view of pollen from a White birch is shown at the right. The grains are numerous. Pin oak pollen is also shown, but in greater detail. The shape is the same. Pollen from a red maple is identical, but notice the semi-transparent octopus object among the grains? That's an important clue. Check out the Norway maple pollen - it too is the same. And note the barely visible semi-transparent worm-like object, which is another clue! White pine pollen is also shown, and it too is the same. It also has the occasional octopus-like growth as shown in the sample. The Silver maple sample also looks identical to the others, including a transparent worm-like object. The blue spruce samples, in spite of being conifers, also have the same pollen. One of these samples also contains both a semi-transparent worm-like and octopus-like object. Finally, there is a lilac sample, which also has identical pollen, and also includes one of these semi-transparent octopus-like objects.

In summary, no matter what tree I gathered the "pollen" samples from, they were identical. And, they were often accompanied by translucent ribbon-like growths. So, the evidence is strong that this enormous "pollen" release was not really normal tree pollen, but white canker fruiting bodies. Still, a finding such as this almost defies belief. But if true, in the coming years, this disease may cause a massive decline in the health of our trees and shrubs.

The Trees and Shrubs Most Affected by White Canker

Unlike almost all other diseases, white canker isn't a choosy disease - it will attack almost any woody plant. In fact, in addition to shrubs and trees, it will also attack vines and plants! It seems to know no bounds.

While it attacks most everything having a woody stem, the set of symptoms and disease severity varies somewhat. Specifically, the most disease prone woody plants appear in the following order:

  1. White Birch
  2. Hemlock
  3. Blue Spruce
  4. Norway Maple
  5. Variegated Maple
  6. Japanese Maple
  7. Red Maple
  8. Sugar Maple
  9. Mountain Ash
  10. Red Oak
Other anomalies are:
  • Viburnum bushes never show the disease on their leaves, but their upper branches get very "rough", and they can get bleeding cankers at their base.
  • Ginkgo trees don't show this disease on their bark at all, but their leaves develop brown spots.
  • Japanese Stewartia trees get and spread the disease like other trees, but it takes a heavy infestation to make it show on their leaves.
Photos of Trees and Shrubs Infected with White Canker

Since white canker isn't formally recognized as a plant disease, there is no literature available on it. Likewise, no lab test so far devised can detect it. Nevertheless, it is real, it behaves like a fungus, and we can characterize it through a variety of detailed observations of its effects.

The evidence supporting this disease grows stronger when we check several plants of a species rather than just one. The evidence grows even more convincing when we make these observations on dozens and dozens of different tree and shrub species. Finally, the case becomes even more airtight when we show that these common symptoms appear over a wide geographic area.

As they say, "seeing is believing". The following list of trees and shrubs contains over 500 pictures that depict white canker. These pictures were obtained from a digital camera, a computer scanner, and a high power USB digital microscope. Each viewing technology enables us to see a different set of disease characteristics.

Each of the pictures referenced in the list below is hyperlinked to a parent picture that shows increased detail. So, if you can't quite make out the details of a picture, simply click on it to zoom in for a more detailed view.

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