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 all 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 disease is primarily a bark disease, meaning that it destroys the tree or shrub by consuming its inner bark, replacing it with a cankerous material. Over time, the tree is weakened as the tree's nutrient transport system is slowly destroyed.

The bark: As the tree's health declines, the responsible diffuse canker within the wood grows. The starving outer bark begins to look sicker, as if it is rotting. Unusually large bodies of lichen move in. The tree tries to heal the damage by growing new "wound wood", pushing out the outer bark, which often results in vertical fissures in the bark. These fissures can sometimes "bleed" sap. In more serious cases, the outer bark will simply fall off in chunks or little pieces.

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 tree is being poisoned and choked, the lack of essential nutrients being sent to the leaves causes them to dry up and lose their natural defenses. They show this by losing their sheen, drying out, drooping, and curling up. Sometimes the leaf tips die first. As this leaf decline happens, other diseases such as anthracnose may move in to take advantage of the weakened leaves. Insects also may move in to feast on the leaves. After the debilitated leaves die, they are shed by the tree, often making it look like fall in mid-summer.

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 and will never support leaves again. The entire tree may be killed in 3 to 10 years.

What the Experts Think this Disease is

Having lived and gardened at this location for 35 years, I'd never seen an abrupt tree and shrub health decline like this. 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 about 2003 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 UNH plant diagnostic lab. They too were unable to identify it, but guessed it was some kind of root fungus.

A lawn care company representative examined the diseased trees and shrubs, and guessed that it was anthracnose, recommending deep root watering and fertilizing.

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 the 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 said anthracnose was especially bad that year, and that the solution was to simply wait it out. So I tried that with my rapidly declining Kwanzan cherry tree. A few months later it totally died.

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 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 to 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 anthracnose, the anthracnose is there because of the weakened condition of the plant. This weakening is 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. 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, and 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. I'm still guessing that it is a fungus or fungus-like pathogen, for reasons explained below.

Test Results Using Various Fungicides

Fungicide: Any agent that destroys or prevents the growth of fungi.
Being unwilling to just sit around and watch my trees and shrubs die, I called a local tree/shrub/plant care company and asked them to spray my yard - something I'd never done before. But, based upon some 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

If you apply one of these fungicides, I'd recommend that you hire a commercial tree care company to do the application since they are trained in their use and have high powered sprayers which can reach relatively high, although not the tops of tall trees. You may want to contact some of the tree care companies listed in the Google Ads within the right margin to check on prices and whether or not they can spray with one of the above fungicides found effective against white canker.

On the other hand, I've found Mancozeb and Propiconazole available locally, and have used Mancozeb myself on small shrubs and trees. Regardless of who applies the fungicide, expect to see results in 7-10 days. Damaged leaves will stay damaged. New growth or slightly damaged leaves should show dramatic improvement. Half of its effectiveness wears off in a day and a half. I've found that you have to reapply about every 3 weeks or so.

The fact that many fungicides were effective seems to imply that this disease is a fungus or fungus-like (e.g., Phytophthora) pathogen.

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.

Where to send in Tree Samples for Diagnosis

If you suspect a disease or insect is attacking your plant, you may want to send in a sample to a 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 - that 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.

Diagnosis Goals & Tools

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 liked that advice, so I spent many hours outside looking at diseased trees. Then I spend 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.

The Goal
Since the experts were unable to pin down a name for this disease, or even agree that it existed, I decided that I was on my own in characterizing this disease. While researching various diseases, I became frustrated in the lack of sufficient diagnostic pictures. If pictures were available, there were only a few, and they might be only of whole leaves. High resolution pictures in color of leaf and bark parts were especially hard to find. Even when pictures were available, they were often only of one or two species of a plant - not enough to determine broad disease characteristics.

Hence, I resolved to publish a large body of detailed, color pictures, on a variety of trees and shrubs having this disease, with the goal of facilitating diagnosis and understanding just how this disease damages the tree or shrub.

The Tools
Where possible, I decided to use tools commonly available to the average curious homeowner. That way my results could be easily verified by others. Those tools include a digital camera to take large-area photos, a modern home computer, a high-resolution computer scanner (2400 dpi) to examine finer structures, a photo editing program to crop and correct pictures, and finally a hand-held USB digital microscope having a 400x magnification (available for about $100). The set of photo evidence produced would be compiled into a document and published on a website for all to see.

These tools would be used to show the disease's effect on the whole tree, the leaves, and the bark. Furthermore, important telltale microscopic structures could also be shown.

Detailed Disease Symptoms

After examining dozens and dozens of trees over a period of years, both on a large and small scale, a common set of disease symptoms has emerged. Many of these symptoms are common to other diseases, too, just like a person having a fever doesn't indicate 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 Leaves both feel and appear to be very dry, as if starving for water. They are often limp, and lose their sheen. 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 likely 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 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 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 the 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 reproductive spores. Since the spores are released in early June, this evidence mostly disappears after that.

Overall, the tree will decline and die, usually branch by branch. As the disease increases in severity, holes may open in the bark and the tree will "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.

Microscopic examination (400x microscope) of the leaves and twigs of an infected plant will often show spores that resemble pollen - especially before spore release time, around June 5th. These spores 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. Spore 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.

Phloem: Inner bark tissue that transports food (sugars) from the leaves to other parts of the tree.
But like mushrooms, these spores 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 consumes the phloem, it both expands (pushing the bark out) and sends out ribbon-like threads to infect other areas of tree tissue.

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.

Why I Named this Disease White Canker

In spite of dozens of tree tissue samples having been sent to several state plant pathology labs, this disease remains unrecognized and unidentified. Test results typically come back pointing to a variety of diseases—or no disease. Yet the similarity of disease symptoms seems to point to a single causative disease.

Since it's awkward to keep referring to this tree disease as "this disease" within this document, I've decided to name the disease after its chief diagnostic characteristics. First and foremost, this disease shows itself as a canker disease. 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 internal tree or shrub tissue under a high-power 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.

The Spores are Key Diagnostic Evidence

In spite of the collection of disease symptoms given above, most people find it hard to believe that they are associated with white canker. For example, some say that the bark splitting is due to winter freeze damage. Others say anthracnose could cause the problems. Other suggest stress due to global warming, too much or too little moisture, or even insects. The basic problem is that while we can see the symptoms, we can't see the disease itself when we look at a diseased tree.

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.
Luckily, this white canker disease isn't totally invisible. But, you have to know where to look. And sometimes, when to look. The key idea is that if this is a spreading disease, it has to reproduce. If it is a fungus, or fungal-like organism, it will most likely reproduce by spores, which are essentially very tiny seeds that are dispersed by wind, rain, birds, people, cars, etc. Since spores are just the "seeds", there must be some growing objects associated with them. We just have to find them.

By chance, I happened to stumble upon these spores while I was pruning an overgrown chokecherry tree. The distinctive fissures in its bark and prior leaf damage indicated that it definitely had white canker. My prior observations seemed to indicate that white canker primarily attacks the bark of a tree. On a hunch, I cut off a diseased-looking branch, placed it on my computer's 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 composed 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.

Next, I began to wonder if these disease objects could be found on other trees and shrubs that showed similar disease symptoms. So I began collecting branch samples from a wide variety of shrubs and trees that my prior investigation had shown to be infected. Sure enough, they all had these disease objects on them. And the density of these objects generally correlated with the severity of the disease. I became more convinced that I was now seeing 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 spores 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 pattern on other diseased trees and shrubs.

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

White Canker Spore Details

It appears that one of the most telltale pieces of evidence for white canker is the appearance of the spores it uses to spread. Here is a summary of what I've found out about these spores:

Spore general appearance
Spores are the most easily seen diagnostic object. Mature spores are milk-white. They are globular 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 spores, appear to be smaller, more globular, and yellow or yellow-green in color.

An individual mature spore appears to be about 50 millionths of an meter (50μm) across, 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 spores are even smaller - often about one-fourth the size of a mature spore. A 10x eyepiece can be used to distinguish the individual spores, but the distinctive shape is only apparent when viewing them at 400x. A large body of spores will appear as a tan-colored area to the naked eye, and may be easily passed off as a dirty smudge.

Spore location
White canker is primarily a bark disease. It's spores are almost always located at branch and leaf junctions on the underside of a branch, about 1 to 5 feet from a branch tip. This wood is generally one to three years old. Spore density can be so high that it totally covers the wood. Unless the wood is heavily diseased, you won't find the spores anywhere else. So, unless you know where to look, and what to look for, you probably won't see them. When to look is also important. They are abundant just before spore release time (early June). Far fewer are seen after that.

Spore hardiness
These spores 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 spores off. I've found these spores 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 spores for some time. The leaves may wither away, but the spores will remain.

Spore host preference
All the above spore observations seem to apply no matter what the shrub or tree. Almost all trees and shrubs seem to have the same spores 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, in northern New Hampshire, 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 spores - and they were present.

Icicle-like Ribbons Among the Spores

Spores rarely appear alone. Wherever they are present, you can usually also find objects which look like long translucent ribbons. They are often three times wider than they are thick. Their slightly bumpy appearance makes them look like icicles. They are about 15 micrometers thick. They have a pointy end, and it can be free or anchored in the bark surface. Sometimes they can be seen to have several twists in them.

Cortex: Inner bark tissue that lies between the outer bark and the phloem. Within branches, it is often dark green in color.
They can be found on the outer surface of the bark, or adjacent to the green cortex just inside the bark. Sometimes multiple ribbons may erupt from one spot. This ribbons allow the pathogen both to grow into plant cells to get food, and to spread the disease throughout the tree.

Technically, these translucent ribbons are referred to as Hyphae (HY-fee). One ribbon is called a hypha. When multiple hyphae branch from a single location (giving a spider-like appearance, it is called a mycelium.

White Cankerous Growths Under the Bark

Spores and their associated translucent ribbons rarely appear alone. If you examine a cross-section of a branch infected with white canker (at 400x), you almost can't fail to notice that the wood just under the bark has what appears to be white waxy substances. There is no distinctive shape to them, and they may be white or light gray in color. Sometimes they form thick layers under the bark.

Regardless of their form, the more of this cankerous material there is, the less green phloem there is, and the more the tree is choked from transporting nutrients. In fact, you can often see this cankerous material pushing out the bark as it grows. Sometimes the tree produces wound wood to compensate for this destruction, causing the outer bark to bulge out even further.

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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