August 2007
By: Russell N. Peters, Plant Pathologist, Certified Arborist

Components of Diagnosis, Part 1: Foliage and Twigs

This is the first part of a three- or four-part series on diagnosis of tree and plant problems. If you spend any time at all diagnosing tree problems, you will likely find different approaches to the process. They may appear different at first glance: however, I feel that any tips or guidelines on tree diagnosis have the same result, and I bet they are all the same basic steps with different language describing the recommended steps. I have used language and a method here that has been successful with many of the new arborists we hire as well as in lectures to college level students of urban forestry.

Before bringing the affected material out to start the process. it will he helpful to have available a few pieces of simple equipment. The most valuable is an inexpensive to moderately expensive dissecting microscope. available in most science supply catalogs. A scope that only magnifies to 50-75 times can serve the purpose beautifully. Having its own light source is convenient. but supplying one is not that big a deal tie- this type of scope. An X-acto knife and forceps are simple tools that can go a long way in helping any diagnosis. Let's begin.

Basically I approach diagnosing any complex problem that is unfamiliar by dividing the tree into four basic regions. starting with the foliage and twigs. This includes but is not limited to observing the last three to five years of growth rate or internodes' growth increments. Be careful here, as different species can have different "healthy- growth rate increments. Species performance will also vary on different sites, in different soils and under different cultural conditions. Inspect other individuals on the site. or area of the same species to see what the average is in that area. Knowing the expected growth rates, under all the different variables, simply comes from experience as well as time you spend in reference materials.

This is the point at which you pull out any reference books or material you normally use. Having one or two picture type reference books is very. important. Having images of other plants with the same conditions can speed the process dramatically. There are a couple of books I always use when diagnosing. There is nothing at all wrong with using these resources. Just be sure to thoroughly read about the origin, description, reported occurrence, life cycle, species normally effected by the organism, and suggested methods of disease management.

What does the foliage look like? Are there apparent abnormalities in the surface of the leaf? There might be what looks like a foreign structure that appears to be originating from below the leaf surface and securely attached to the leaf tissue itself.

Structures like these likely arise from a fungal type of leaf pathogen that enters or infects the foliage in the early spring, and what you are seeing is a reproductive structure produced by the pathogen that normally develops spores or similar types of structures in order to begin to spread, possibly to adjacent foliage. other parts of the same tree, or additional trees. This would only occur successfully if environmental conditions favor continued infection of the pathogen. If it was still cool and available moisture was present, then the pathogen might be increasing in coverage.

Not all fungal types of organisms require cool and moist environmental conditions to spread, germinate and infect new foliage. Case in point would he Enomosporium sp. that infects foliage of photinia and hawthorn sp. This organism indeed spreads and infects new foliage at a higher rate during cool and moist conditions. It does not exclusively move during that time, but also during periods of warming air temperatures.

The fungal leaf structures visible on the leaf surface may be sparse, with a small number scattered across the leaf surface with the area surrounding the structure usually discolored, appearing yellowish. This area commonly progresses to a brown color appearing necrotic as the tissue dies. These areas may grow together into a continuous leaf surface that appears distorted and certainly not normal.

Did the leaf develop normally in size, shape, and color or does it appear larger or smaller than normal for that particular plant? This may quickly include or eliminate chemical injury as a possible diagnosis.

The point at which you decide that the problem is biotic (caused by some type of biological agent) or abiotic (caused by cultural or environmental effect) is usually in the beginning. Many times you may not have enough information to separate the problem into one or the other but by this point, you certainly should.

A plant can have a fungal leaf infection without necessarily having prominent structures visible on the leaf surface. At this point in the diagnosis, you may already have a couple of foliar disease candidates that you suspect being the problem.

If you get close to something you suspect, as I have stated before, it is just as important to eliminate things that it is not, for as you do this, the list of correct candidates begins to shorten rapidly. So do not feel frustrated if you think you have reached a dead end; instead, you have actually reached a point at which you can refer to reference material!

Let's say you have determined that it is fungal and isolated to the leaf. Most quality reference books can do the rest. You identify the species of plant/tree, that your problem is fungal and only on the leaf, then you look under leaf diseases of that species, and there should be an entire section in a good reference book. Now you start comparing images of other plants or the same plant that you believe has the same problem. Many of us find the problem here.

It may be that the leaf is expressing abnormal symptoms and no type of structures can be found on the leaf surface; the symptoms being expressed are developing from something back from the leaf. That is to say it is originating on the twigs, limbs, branches, trunk, or root system. We will only consider other things that may originate on the twigs right now.

This is where insect infestations may be the origin of the leaf symptoms. Look for scale insects at and along the twigs. Some scale will be visually apparent as small semi- roundish bumps that can have some reddish or tan color.

There are other scales that placing the twig under the dissecting scope may reveal: tiny and small or obscure grayish colored structures, very flat along the twig. These can be obscure scales common on many oak and pecan species. This is where the reference books can solve the rest for you; look in the section of scale problems on that species.

This is also where you might indeed discover tiny structures very flat to the twig surface that would be fungal lesions often similar to what you saw on the leaf surface. If so, then investigate twig blights in the reference books on that species.

Look also for insects that bore into the twigs. Small weevils or beetles can commonly cause leaf abnormalities by cutting off the flow of moisture and nutrients to the leaf. The presence of boring type insects will leave evidence of tiny holes where they were present. You may get very lucky and the insect is present on the sample, but more often just the evidence or damage is present. It will be consistent regardless of what species they show up on, due to the presence of uniform type holes corresponding to the size of the insect.

Again, reference books can take over here, and again by comparing images in the books you can find the pest causing the problem.

You can see that I am recommending to heavily rely on reference material, and the purist would not be happy with me. The fact of the matter is that most arborists have not had pathology courses or a strong disease background. Reference books have their limitations but they are the one resource we all utilize. Why not, if it's going to help you determine what your problem is? I will list those reference books I recommend at the end of the last part.

This concludes the first part. Part 2 (next time) will include the origin of your problem further back on limbs, branches and scaffold branches.