Oak, Red #1
This Red oak sits on the corner of our lot and has been decline for several years. Being unhappy with several vague diagnoses, I contacted the Cornel plant pathology lab. They suggested I send in a bark sample. Unsure as to exactly how to do this, and not wanting to miss any diseased wood, I decided to take a large (6" by 6") and deep sample. I used a hammer and wood chisel to obtain sample bark around several weep holes.

The following pictures, taken September, 2006, show the sampling process.

Oaks having White Canker disease often have holes in the bark that they bleed sap from. Here are three such holes. I decided to obtain a bark sample from this area. To begin, I marked the area to be sampled with blue tape. Note the inch marks on the tape.
All the outer bark was removed here. The weep spot areas seem to be decayed. In fact, the decayed area appears much bigger when the outer bark is removed.
Finally, after much effort, I was able to also remove the inner bark too, exposing the sapwood. As this picture shows, the disease has burrowed through all the bark and has continued into the heart of the tree. The wood is black around the area infected the most.
The Cornell diagnosis was that this tree had a Phytophthora disease (species unknown).

I resumed my investigation in May of 2008, using my scanner to more closely examine parts of the tree, as shown in the following pictures.

This live oak branch is also about 1/4" thick. The top branch junction was just 6" from the branch tip, with no spores between it and the tip. The spores are so thick at the branch junctions and leaf scars that the area appears almost crusty.
This cross-section of the branch shown in picture 2 was made with a 2400dpi scanner. The black spot appears to be a disease infection channel. The spot's location suggests that this tree became infected about 5 years ago (around 2003), which is about when I started noticing disease symptoms on almost all of my trees and shrubs.
This is a dead branch from the red oak tree in decline. Not only is there a high spore density at the branch junctions and leaf scars, but the spore density is moderately high between these points too, making this tree a prime transmitter of disease.

This branch segment was about 1/4" thick, and was located 12" from the branch tip. There were virtually no spores from this point to the branch tip.

The following set of pictures wase taken in early October 2008, using a 400x digital microscope. Since I believe the spore release took place shortly after the above pictures were taken, I didn't really expect to see many spores left. The first two pictures are of the top of a leaf.
Adjacent live and dead spots on the leaf top. The dead spot looks like it could be made up of white canker particles.
In general, a pretty boring leaf top surface. As shown here, there were scattered small pieces of white canker, and a few of these spider-like objects.
Since it's just as easy to check the leaf bottom as the top, the following pictures show the bottom of the leaf at 400x.
The leaf bottom had numerous spider-like objects scattered about its surface. But perhaps of even more relevance was the presence of white canker spores (red arrows), along with their visual characteristics. The one at center-bottom appears halfway through its embryonic stage, since it's about half yellow in the center. The one on top is almost through its embryonic stage since there is only a bit of yellow showing through its two halves.
The spores and spider-like objects get more numerous the closer you get to the leaf's stem. This view is next to the stem. Of interest here is the variety of development stages of white canker spores. One (red arrow) is pure yellow and just starting to differentiate. Three more (blue arrows) are about half-yellow and hence half developed. Another one (yellow arrow) is almost totally developed, but still has a yellow stalk. Finally, one (green arrow) is fully developed and totally white.
This is another area of the leaf, close to the leaf vein (in the upper-left corner). Notice the group of developing spores (red arrow) right next to a "spider" object. Each spore is about half-way through its developing phase, still having a yellow globular mass within its two halves.
White canker often likes to hide within the body of a leaf. The following set of 6 pictures check out that possibility via leaf cross-section views at 400x.
There are two patches of white canker near the center of this picture that run from the top to the bottom of the leaf (red arrows). But note the internal gray blob of canker that seems to be giving rise to the spore on the bottom of the leaf (blue arrow). There is also a trail of canker (yellow arrow) from this spore to the right and then up to the top surface of the leaf, where there is another particle of canker material.
The left-center part of this leaf's cross-section is of the most interest, as there is a long "tail" of canker descending from the leaf's bottom (red arrow). But where this tail enters the leaf, it gives rise to two spores, one of which appears to be cut in half (blue arrow). The stream of canker then continues on left to the top of the leaf. Again, check the spores - it appears as if one has a white root which intersects with the gray canker blob. The intersection forms a tiny yellow spot (yellow arrow). The question is... did the canker give rise to the spore, or did the spore land on the leaf, infect it, and then give rise to the canker?
This spider-object appears to have a root (red arrow) going directly into the leaf. The leaf tissue disturbance hints that the root goes all the way through to the top side, emitting a green blob (blue arrow) on that surface.
There are internal blobs of white canker material on the right and left, that are connected by a tube of white canker (red arrow). A leaf bottom view wouldn't show this, and a leaf top view would only show two small disconnected white spots. This cross-section shows there is a lot of canker within the leaf that isn't visible.
A side view of one of those spider-like objects. The twisted shape and aspect ratio of the top projection (red arrow) match those of the hyphae of white canker.
This is a cross-section of the area around a leaf vein. Going by past experience whereby light-gray-translucent and clear material is canker material, this leaf vein is about 80% consumed by white canker. It's not enough to kill the leaf, but it is enough to weaken it severely.
The above set of pictures make a strong case for a white canker infection. But experience has shown that twig cross-sections provide the best evidence for white canker infections. The next set of 7 pictures are all cross-section views of a 1/8 inch twig made with a digital microscope (400x).
A good solid confirmation of white canker infection. Particles of white canker have invaded all of the phloem. A patch of canker has colonized the bark (red arrow), but the tree has grown wound wood around it, sealing it off. Two rays appear infected (blue arrows) too, transmitting the infection to the xylem in the center of the tree.
Once again, most of the phloem has been infected, and it has been so weakened that the bark is separating (red arrow) from it. The bark is also infected, so much so that it is turning white (blue arrow).
White canker has so infected the phloem that it is beginning to shrink and disappear. The canker is also now spreading into the xylem, destroying it too. And it has even managed to jump to the outer bark, replacing it with clear white canker.
Very severe white canker damage is shown here. The green cortex is heavily infiltrated. The bark in the lower-left corner has been eaten away and replaced with canker (red arrow). The phloem, where it meets the xylem, as turned to a waxy light gray-green color (blue arrow). All this canker growth has caused expansion, separating the outer bark and causing the xylem and phloem to separate (green arrow). Lacking nourishment, the xylem will no doubt weaken and die. The branch will then become brittle and easily break off.
This really isn't a true cross-section, but about a 20 degree slice into the side of the twig in order to get a better idea of the true shape of this canker. As seen here, the canker appears as cloud puffs along a string (probably a hypha, actually) that runs mostly parallel to the twig axis. Each tiny puff is probably a killed tree cell.
This view is even further along the 20 degree razor cut. It's clear that the canker much prefers the green phloem layer, but also grows more weakly and diffusely in the yellow xylem wood.
This picture continues further down to the interior of the twig along the 20 degree (to the twig axis) cross-cut. Most of the center of the picture is the xylem, and the "puffs of white" have turned to mostly streaks of white. That's probably due to the infective hyphae being channeled along the vessels of the xylem, using them as a sort of "road" to infect other tree tissue.
In summary, while white canker is present in the oak leaves of this tree, the leaf surfaces were only marginally helpful in diagnosing this white canker infection since the vast majority of the canker lies within the leaf. Therefore, a microscopic view of a leaf cross-section more clearly shows the white canker infection. Nevertheless, the best diagnostic evidence is obtained by examining the cross-section of a small twig. There the infection is concentrated just under the bark, at the junction of the light green phloem layer and the dark green cortex layer. The white canker substance is easily seen there since it disturbs the natural growth so much.

Oak, Red #2
This large red oak stands right in front of our house. It probably is 20 inches in diameter and is about 60 years old. Yet for the past 5 years it has been in decline. More branches die every year. The bark has developed fissures and cankerous material is now (2008) pushing out of a large vertical split shown in picture 1. These pictures were taken in early September 2006.
This long vertical bark fissure seems to expose a peanut-butter colored interior. Splits like this are very typical of White Canker. In addition, the bark will have an "unhealthy" look to it, and lichen will more easily grow on it.
This tree was rotting under its bark, indicated by wet spots (blue arrows). I pulled off some of the decayed bark a year or two before this picture was taken. Here you can see where wound wood was trying to heal the damage, but the wound wood itself is unhealthy. This tree is continuing its decline.
A leaf from the tree showing signs of White Canker. The principle sign here is the puckering of the leaf - a common characteristic of this disease. The yellow smudges may be White Canker spores, but at the time this picture was taken I didn't have a microscope to verify that. The White Canker probably significantly weakened the leaf, so that it more readily suffered from insect and fungal attacks.
The following pictures were taken in early June 2008. The first two were taken with a high resolution scanner. The remainder were taken with a microscope (400x)
The branch junction shown here is from a quarter-inch diameter live branch, but one in decline. This junction was the first branch junction from the tip of a branch (18" in) and was about 10' high. The entire branch was completely clear of all signs of infection except for the center of the branch junction facing the ground, where there was a high spore population, as seen here.
This is a 400x picture of that area in picture 4 having a high spore density. At this magnification it is evident that the light yellow coloring of the spore patch is due to many milk-white mature spores sprinkled with yellow embryonic spore tissue. (Note: a major white canker spore release took place a day or two later.)
Here's another picture taken within that same spore patch. The blue arrow points to what may be an embryonic spore evolving into a mature white spore (the black spot on it may be indicative of the transformation taking place). The picture also shows something very common on infected leaves - a white spider-like object that appears translucent. This spider object appears to have a some spores associated with it (green arrows).
Here is another interesting picture from that same spore patch. The green arrows appear to show two places where embryonic yellow spores are currently transforming into mature white spores. Note the black spot on the side of one of the white spore hemispheres.
This 3/4 inch diameter branch segment was about 6' from the Branch Tip. While this segment hosts an enormous number of spores (click the picture to get a close-up view), these spores seem to be concentrated at the branch junctions on one side of the branch.
When oaks are badly infected with white canker, they will often develop a vertical protruding canker that runs from a foot or two above the ground to about the 6 or 7 foot level. This oak had such a canker. Using a chisel, I chipped off a piece of it measuring about 1/2 by 3/4 inch. Physically, the canker piece seemed to have the texture of weak particle board. This is what it looked like when placed on a scanner and scanned at 2400 dpi.
This is what the surface of that chunk of canker material looks like under a 400 power microscope. Note the big white blob of cankerous material under the bark.
This is what the interior of that chunk of canker material looks like under a 400 power microscope. It is a waxy, translucent material, and seems to be a mixture of wound wood and white cankerous substance.
In summary, this tree too is clearly infected with White Canker. The evidence is in the puckered leaves, the distinctive bark fissures, the unhealthy bark, the spore colonies on the bark at the branch nodes/junctions, and the characteristic spores themselves.

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