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>/i>. 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.

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 deepens, the overall appearance of the plant will give the appearance that it is lacking water or nutrients. The plant will often have a "droopy" appearance. The casual observer will think that food, water, drought, wetness, heat, or cold is a major contributing factor.

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?)

Anthracnose: Any of several diseases of plants caused by certain fungi and characterized by dead spots on the leaves, twigs, or fruits.
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 throughtout the plant. This lack of water will cause the leaves to dry out, losing their sheen, drooping, and curling 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 proper 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!

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 defnitive 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, he and 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, 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.

Pathogen: An infectious biological agent that causes disease or illness to its host.
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 that there was one pathogen that was the root cause of it.
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 mould. 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.

I had a lab specifically check for this disease - it 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.

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, 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 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.

My Diagnostic Tools
As I mentioned earlier, when you examine diseased plant tissue, its 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!

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 vast 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 such 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). The cost was about $100. I would have gotten one with even higher power, but 400x is about as powerful as they come.

In addition, I supplement this with a USB flatbed scanner that can scan at up to 3200 dpi. I sometimes use it for scanning entire plant leaves.

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. But most any good photo editing software will do.

Another simple, but crucial tool is a hobby vise. I use its jaws to grip samples. The jaws are flat on top, which is perfect for resting the USB microscope on when gathering microphotographs. That resting platform is crucial since the depth of focus at 400x is extremely critical. One of my hands holds the microscope down on the vise jaws while the other hand clicks the mouse to snap the photomicrographs.

Later in my diagnostic work I added what I also consider to be a very important tool - the free Microsoft program called ICE (Image Composite Editor). I've found that while 400x microphotographs 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.

Here's a perfect example. I want to get a cross-section of a twig. So I obtain a twig, cut it with a razor, and then mount it in my vise with the cross-section facing up. My USB microscope is connected to my computer and Photoshop Elements is used to view the microscope's image. As my left hand moves the vertical microscope over the cross-section, my right hand left-clicks the mouse to capture successive, overlapping images. I might capture anywhere from 1 to 200 overlapping images this way. Then I save each of them to a file folder I created. Then I call program ICE to stitch them all together into a "panorama" view. This takes anywhere from a few seconds to 10 minutes. The resultant single image is then read back into Photoshop Elements, rotated, cropped, and color corrected if necessary. I then describe the photo content using Photoshop Element's EXIF editor. If you're technically saavy, you might say I could use the stitching operation in Photoshop Elements 11 instead of ICE to do the job. I tried that, but it often hangs on over a dozen or so images. And sometimes I've found it distorted the result. ICE never distorted the panorama, has been very reliable, and can handle the stitching of over 200 photos! One other tidbit of insight - I've found that to get a good cross-section, limit the twig diameter to about 1/8" or you will need more than 200 photos to cover the cross-section. Plus, my hand tends to get very sore after carefully moving it around to obtain over a hundred orthogonal microphotographs! Plus, it's very difficult to make a clean razor cut on a twig greater than 1/8" in diameter. A full panoramic picture of a 1/8" twig can take a total of about 45 minutes. That's a lot of time, but the results are often breathtaking!

With this set of tools, I'm able to gather and document an large amount of information on plant diseases. Later on we'll see a lot of these photos.

Unaided Eye Disease Symptoms

After examining hundreds and hundreds of plants, shrubs, and trees over a period of years, a common set of disease 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 this particular disease.

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 lost much of that interesting color, which will turn somewhat 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. The leaves will both feel and appear to be very dry, as if starving for water. They are often 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.

Phloem: Inner bark tissue that transports food (sugars) from the leaves to other parts of the tree.
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.

Microscope Aided Disease Symptoms
As covered earlier, 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 on 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 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 (a fungus) 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 cycles 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 400x handheld 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 of this disease? 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 investigaton 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. The white canker fruiting body These white canker fruiting bodies have these characteristics:

  • Shape: Mature fruiting bodies are globular, milk-white, with an upward-facing cleft, making them resemble a 3-D, fat, and upward facing pac-man. Sometimes their color and shape remind 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 body of fruiting bodies will appear as a tan-colored area to the naked eye, and may be easily 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 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, when white canker fruiting bodies are shed is just about the same time that white pine pollen is shed. So I looked for pictures of white pine pollen on the Internet, and also examined sacks 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 phytophthora 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 white canker fruiting bodies are distinct from white pollen 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.) Icicle-like Ribbons Among the Fruiting Bodies

While white canker fruiting bodies most often appear to have no support, they are 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 15 micrometers and a width of three times that size. Their slightly bumpy appearance makes them look like icicles. While hyphae are usually burried in the phloem and bark, 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 pointy. 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.

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.

In fact, I've found that all you need is either a single leaf or a twig that has a diameter of about 1/8" or less. Larger diameter twigs yield no 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! While this fungus is present in all parts of the leaf, it tends to congregate where its food supply is most plentiful - near the leaf veins, and in particular near the leaf midrib and stem base.

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 is white (or light gray), and most healthy internal plant tissue is green. And where some internal plant tissue (the xylem) is white, this white tissue has a distinctive patterning, and white canker hypha covers up this patterning with opaque white (or light gray) blobs. Therefore, no diagnostic staining is necessary.

An issue of practical significance is the size of the twig cross-section. To more clearly see all the shapes and colors within a twig cross-section, a nice clean cut is necessary. So instead of a pruning tool, I use a single-sided razor blade and cut the twig perpendicular to its length. Through experience, I've found that about the largest twig that I can cut with this method is about 1/8" in diameter. Generally a twig of that size will often include a layer of bark, which is also helpful in diagnosis.

Using a needlenose pliars to assist me, I then mount the twig vertically in a vise so that it extends up about 1/16" to 1/8" from the jaws. I do this for two reasons. The first is that I don't want images of the jaws interferring with the twig pictures (their distance away puts them out of focus), and the second reason is that I don't want the pressure of the vise jaws to distort or crack the internal twig structures. Futhermore, I find it somewhat important to make sure the flat face 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.

While the resulting views small areas of these cross-sections can be very informative, after awhile a certain level of frustration can set in. I'd compare it trying to comprehend a picture of something by looking at it through a peephole, i.e., it feels so much better to be able to look at the "big picture". I've found that this is possible, but unfortunately not quick and easy.

What one needs is a "panorama" of all the microscopic images making up the twig cross-section. I do this by taking multiple, overlapping, images of the entire twig cross-section, saving each one as a JPG file in a dedicated computer file folder. When finished with this operation, I call up a program to "stitch" them together into one big panoramic picture. I've found that Microsoft's free program called ICE (Image Composite Editor) does a wonderful job. (I've tried Adobe 11's stitching utility but found it was very slow, often hung, and sometimes distored the final panorama.) But there are several very practical issues to consider when making these panoramas. For one, I gather the microphotographs within Adobe Photoshop Elements 9, and it has a limit of being able to store 200 of these photos. Another very important isssue is that, for proper stitching, these individual photos must overlap by no less than 25 or 30 percent. Furthermore, when gathering these photos, you can't rotate you hand much at all, since rotated photos also often don't stitch. Finally, keeping your hand steady on the handheld microcope, moving it to ensure proper overlap but being careful not to rotate the microscope, and doing this to obtain 100 to 200 individual photos can be a painstaking and frustrating job! It's especially frustrating when you've taken the time to gather over 100 photos and then discover you made a mistake and only a small fraction of them will stitch together! Yet, when the generation of a twig-cross section is successful, the result can be both very stunning and informative.

Test Results Using Various Fungicides

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 the cause of your tree and shrub decline is due to white canker, you will no doubt want to get rid of it permanently. Unfortunately, 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 is the easiest and safest way to go. But it is also the most expensive, costing anywhere from $50 to $100 per treatment, and you may need 4 or 5 treatments during the growing season. Each treatment generally takes about an hour or so. A commercial firm generally has a large body of customers, and the firm may not be able or willing to spray the particular fungicide needed to control white canker. Or, they may be backed up and not be able to apply the fungicide treatment for 8 weeks or so. Or, to prevent "fungicide resistance buildup", they might alternate an effective fungicide with one that is not effective in the treatment of white canker. On the other hand, when they treat your property, they usually apply a mix of chemicals. This mix usually also includes some insecticides in addition to a fungicide or two. Applying the fungicide yourself has the advantage of being low cost, as you only need to obtain a sprayer (available from local home and garden centers for around $30, and the fungicide concentrate. The effective concentrated fungicides are only available at some home and garden centers, but can be ordered online (e.g., Amazon). Some, like propiconazole, come in various concentrations (e.g., 2% or 14%). While you can choose to spray on your own schedule, these fungicides can be dangerous if not handled properly. I've been cautious when working with them and so far haven't had any problem with them. Another point - so far I haven't found any harmful effect of applying too much of a fungicide, so dosage doesn't seem to be a problem.

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. The problem they have is with tall trees - their pressureized spray equipment can only reach up to about 25 feet. And then they recommend that you stay off your sprayed property until the spray drys.

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 this disease 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 one 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.

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, they will improve in health with time. Since the white canker fungus kills the internal plant tissue, it will take time, sometimes months, for the plant to heal that tissue, much as it takes your body 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!.

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.
  • Over the years, this decline 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 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. Eventually, all the needles are lost. At some point during this decline, the owner of the tree becomes so discouraged that the tree is cut down.

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 (Cytospora kunzei)
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.

Comments: Good match with the first symptoms. But no white resin as found. More importantly, these black fruiting structures were also not found.

Phomopsis (Phomopsis occulta)
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.

Comments: Good match with needles turning brown and dropping. No match on resinous exudate. 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.

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.

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

Diplodia (formerly Sphaeropsis)(Diplodia pinea)
The most conspicuous symptom of diplodia blight is brown, stunted new shoots with short, brown needles.

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

Spruce Gall Aphids
This insect causes the formation of conelike galls on developing twigs, which deform, stunt and usually girdle them.

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.

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 strengthend 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. And since this disease seems to be very common, it probably produces a lot of them. 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 - 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 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 seriusly 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 1 - Blue Spruce 1 twig cross-section

What the White Canker Fungus Looks Like
As they say, "a picture is worth a thousand words". Figure 1 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 from 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. Be sure to click on this picture to expand it so that it almost fills your display screen.

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 the 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 orangish-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 hypha that are growing out of the cankerous material within the twig.

Figure 2 - Dead Blue Spruce twig cross-section

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!

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 2 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 3 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 3 - Blue Spruce 2 twig cross-section

Figure 4 shows a 400x photo of a small section of bark. What you see is a field of white canker fruiting bodies intermingled with the hypha 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 4 - Small section of Blue Spruce Bark

Figure 5 - 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 5 shows the base of a needle. Note that the stomata are white, not black, so none of the common blue spruce diseases are present. Figure 6 shows the middle of a needle, while figure 7 shows the tip of a needle. As you can see, there is little indication of white canker in these external views of a needle.

Figure 8 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.

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.

Furthermore, as the cross-section of a live branch shows, a tree branch's vascular system can be 95% blocked, and the branch and its leaves/needles can still remain alive. It's a tribute to the resiliance of trees in the face of serious disease.

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. The disease 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 the disease. Our spring fungicide spraying arrested the disease, permitting the new growth to finally thrive. It has been healthy and growing ever since.

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.

Other Objects Associated with the Spores and Cankerous Growths

While perusing the bark of a tree infected with white canker, you may occasionally find other strange objects. Some look like smooth pearls several times the size of a spore. Some are wine-red and others are light-gray. These may be Chlamydospores. A chlamydospore (clam-EE-doe-spores)is a survival spore produced by fungi and 35 of the 75 described species of Phytophthora. It is produced directly from hyphae. Chlamydospores are really swollen hyphal segments, and are globular in shape. They form under stress as a "resting phase", resuming infection when better conditions occur.

For an excellent article on Phytophthora, see:

How the Spores Spread

Recently (in the days 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 was 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 spores 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 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. But 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. But 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 disease spores. If true, this disease is about to cause a decline in our tree and shrub health as never seen before.

The White Canker Disease Cycle

Given all the preceding information on white canker, a description can be given of how the disease progresses

Spore infection
A tree or shrub becomes exposed to numerous tiny airborne white canker spores in June. They may arrive via the wind (e.g., during a spore release day in early June, or via a nearby infected tree), via the feet of a bird, or via a load of infected bark mulch. The spores enter the tree via a bark opening (pruning cut, broken branch, bark wound), young bark, or the underside of a leaf (the topside seems pretty well protected). Once attached (probably electrostatically), a spore sends down a "root" into the bark or leaf, getting nutrients from it. Ribbon-like growths (hyphae) then spread between the cells of the tree tissue. At this point in time, only a few extend out of the bark or out of the leaf surfaces, since it's growing time, not reproduction time.

Xylem: Core tissue that provides strength and transports water and minerals from the roots to the leaves. Sometimes called sapwood.
The hyphae concentrate their attack on the phloem, the source of rich nutrients coming down from the leaves. There they engulf and consume the phloem plant cells, converting them into blobs of a wax-like substance - the white canker. Initially, this causes internal pressure. But after the canker has been created, it shrinks, leaving voids in the tissue. Having consumed much of the phloem, the canker expands outward to attack the xylem, and especially the dark green cortex. Finally, it goes after the bark, replacing it with canker material. Twig cross-sections illustrate this process, which continues throughout the growing season.

Springtime is white canker reproduction time, so white canker uses it energy reserves to build enormous number of spores. Their DNA causes the hyphae to grow and generate them whereever they hit air. That's ususally on the surface of a leaf or bark, but it can be in a void under the bark! The new/embryonic spores start out as a small yellow blobs - in the color and shape of an egg yolk. They then develop into the milk-white mature spores. They grow principally at branch and leaf junctions facing the ground, mainly on wood that is from 1 to 3 feet from the branch tip. These locations have the following advantages:

  • Branch tips are generally high up where the wind makes for a good dispersal agent
  • Branch tips are good perching sites for birds, which can transport the spores on their feet
  • The underside of a branch shades the spores from drying out
  • The underside of a branch collects moisture, feeding and preserving the spore

Bark splits
As the white canker blobs grow in size, they send out transparent ribbon-like structures (hyphae). These hyphae push through he wood and even the vessels that transport nutrients. As they do so, they engulf and consume tree tissue - phloem from under the bark and leaf cells, converting it into canker. The new canker material pushes against the existing tree tissue, causing pressure to build. If this canker primarily grows on one side of a leaf, it can cause the leaf to pucker or curl. If the canker grows in the phloem under the bark, it can cause the bark to bulge out, forming what looks like fissures from the outside as the outer bark is pushed aside. Bark is normally dark brown in color. White canker is white. Therefore, bark infiltrated with white canker appears light brown in color, which can often be seen at the base of a split.

If canker growth is especially vigorous, the outer bark and cortex will both rupture, allowing nutrients from the phloem (and possibly xylem) to bleed out. When this happens, lichen that is normally pewter green turns a beige color, betraying its presence. When the bark of a tree displays these beige-colored lichen areas, it means the tree is already in serious decline due to extensive white canker growth under the bark.

If a tree is healthy enough it will try to resist these canker-caused bark breeches by growing wound wood around them in an attempt to contain the infection. Since the white canker isn't killed , it now becomes a survival race for the tree, with the visual result being a slowly declining tree that is increasingly distored in shape by the growing internal canker.

Overall decline
The tree is now in a life-and-death race, trying to heal itself faster than the infection damage occurs. But it's often a lost cause. The growing hyphae and their wax-like growths take up an increasing amount of the tree's nutrients, while secreting cell poisons and increasing the internal pressure. The cankerous growth also invades the tree leaves. The tree's leaves will wilt and/or pucker up, as if they were starved for water. If its early in the season, they may not grow to full size and/or may die in the process. The canopy will thin. The bark, initially stretched, now begins to die, and may even fall off. The sapwood and heartwood are weakened, allowing branches to more easily break off and potentially cause property damage.

Opportunistic diseases
The weakened tree becomes a perfect target for opportunistic diseases. Leaf diseases, such as anthracnose, often move in. A variety of lichens now find the tree an attractive target, giving the bark a green look. Insect damage increases. The tree may die branch by branch, or whole areas of the tree may thin out first, then die. Branch die-off is due to this infection tending to cluster at the branch junction in some species.

To the casual observer, the tree may appear generally healthy, while in fact the tree is slowly being destroyed from the inside. But this battle takes time to wage. As it does, some of the above signs begin to appear. If a concerned homeowner sends in parts of the tree for disease diagnosis, the result may be an identification of these opportunistic diseases along with a recommendation of "don't worry, this is a minor disease that will go away by itself". It won't.

As this decline continues, the white canker is going full-tilt to reproduce itself. Its strategy is to generate millions of spores as high above ground as possible. Hence, it creates them within a few feet of the branch tips, facing downward, near branch and leaf junctions. Here they get maximum moisture to grow. Not only that, but they also get maximum wind turbulence and mechanical stress, so the spores can be easily dislodged, picked up by the wind, and transported far and wide. As an added bonus, these spore producing branches are good bird perching sites, so the feet of a bird can also serve as transportation. If you're a spore, life is good!

Slow death
In fact, from this disease's point of view it gets even better. The tree or shrub isn't killed instantly, leaving plenty of time for disease reproduction. It may take from 3 to 10 years, so that people don't really notice. And if they do, they'll probably just ascribe it to some other opportunistic disease, too much watering, too little watering, global warming, root damage, physical damage, etc. Hence, they won't do anything about it. When it dies, they'll just cut it down and replace it with another one… and the cycle will repeat.

By evolving this particular disease, nature has come up with an incredibly efficient tree and shrub destroyer!

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. Norway Maple
  2. Variegated Maple
  3. Japanese Maple
  4. Red Maple
  5. Sugar Maple
  6. Mountain Ash
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 get a bigger view.

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