Forest and Shade Tree Pathology

Concepts of Disease and Names

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Forest pathology is the study of tree diseases.

Professor What do you think a disease is? One textbook says: “Any deviation in the normal functioning of a plant caused by some type of persistent agent.”

What is normal? What if all the trees in a population have a particular fungus causing leafspots? Is that then normal, and thus not a disease?

What is “persistent?” What is the difference between injury and disease? You should know that insects are traditionally excluded from the concept of disease, though some are persistent and their interaction with plants fits most definitions of disease. Nematodes are traditionally included.

Do genetic defects and nutrient deficiencies fit the definition? For these cases, you may want to add to the definition the phrase, “or condition.”

Disease triangleAlthough some consider that traditional definitions of disease imply that the pathogen is all-important and leave the impression that the pathogen and disease are the same thing, the traditional disease triangle seems to cover this problem for all sorts of diseases. It is a useful, time-tested model of disease taught in most introductory courses and commonly used conceptually by pathologists. It is a useful model of disease because it emphasizes the interaction of the environment, a pathogen, and a host (suscept) to produce disease. It applies even to diseases where environment is especially important or multiple pathogens are involved (sometimes called 'declines'). It also emphasizes that disease and pathogen are not the same thing.

The length of each side represents the relative favorability of that factor for disease and contributes to the triangle area, which represents the overall amount or severity of disease.

Names of diseases

Diseases do not have formal latin names like organisms, but most do have common names. These names may be taken from the host, symptoms, pathogen, and/or the kind of disease. Sometimes, part of the latin name of the pathogen is included in the disease name. If so, it is never italicized, but if the genus is used, it is always capitalized. Disease names derived from country names are capitalized as usual in English. Examples:

The most serious disease of southern pines is fusiform rust.

The greatest tragedy in American forest history was the devastation caused by chestnut blight.

A major disease of conifers in the northern hemisphere is annosum root rot, caused by Heterobasidion annosum.

Disease names like Swiss needle cast and Dutch elm disease are not much appreciated by the Swiss and the Dutch!

Both conifers and hardwoods are often infected by Armillaria species, which cause Armillaria root rot. The same disease is often called shoestring root rot because the fungus produces black, root-like structures.

In addition to diseases, there are names for types of diseases that are similar. They differ in the particular host or pathogen and often in details of infection, etc., but follow a general pattern that can be used in thinking about the diseases as a group. Here are some examples:

Stem decays are often the most serious diseases in hardwood stands.

In western conifers generally, it is considered that mistletoes are the most serious diseases, but root and butt rots are probably a close second.

Types of diseases

These can be divided up several different ways. A fundamental division used in the book, Tree Disease Concepts, is:

The first two should now be clear, the other is covered in a later page. These, especially, biotic diseases, can be further subdivided by type of pathogen.

Another common way is by tree part affected, modified by considering certain types of agents separately:


A pathogen is an agent that causes disease (“path”-“gen”). It can be living (fungus) or non-living (pollutant). In common parlance, it is often used just to mean living agents. The term doesn't really apply to deficiencies. Here are the main kinds of biotic pathogens:

Names of fungi

Names of fungi, as with other organisms, consist of the genus followed by the species (“specific epithet”). The genus is capitalized, the specific epithet is not. Both should be underlined or italicized. Names of higher taxa (family, order, etc.) are always capitalized but never underlined or italicized. Here are some examples:

Nectria galligena is in the division Ascomycota.

Some fungi have common names, but not many. Common names may be capitalized or not, but they are never underlined or italicized. Examples:

The common name of Fomitopsis pinicola is “red-belt fungus” because it usually has a reddish margin.

Phaeolus schweinitzii has a velvety surface and looks like a brownish lump on the forest floor. It has common names like “velvet-top fungus” and “cowpie fungus.”

Pathogens are not diseases

A disease and the pathogen that causes it are two completely separate entities. Always be careful to distinguish them. Being sloppy in language leads to sloppy thinking. Equating them leads you into the trap of focussing too much on the pathogen and ignoring the role of environment and host in development of disease. Exceptions are rusts and powdery mildews, terms used for both the fungi and the diseases they cause.

The following are examples of what NOT (frown) to do:

frown A serious disease of Douglas-fir is Phellinus weirii.

frown Leptographium wageneri is a root disease spread by root to root contact.

frown Stem decays produce basidiospores in conks.

frown Make a list of diseases, with the disease name column labeled “common name” and the pathogen column labeled “scientific name.”

Angry professor

“You must follow the rules for naming fungi and diseases. I'll be watching!”

“Fungus” is singular; “fungi” is plural. Don't confuse them!

Signs and symptoms

Signs: physical appearance of pathogen. Anything you see that is primarily made of pathogen tissue can be called a sign. Examples:

Symptoms, on the other hand, are alterations in the appearance of the host due to disease. You should know about symptoms like chlorosis and necrosis. Examples:

Funny picture “I need another chip so I can learn this stuff!”

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Last modified 25 Nov, 2007