What is a Tree Disease

Forest pathology is the study of tree diseases.

What do you think a disease is? Some textbook definitions:

  • Any deviation in the normal functioning of a plant caused by some type of persistent agent. ​[2]​
  • Any malfunctioning of host cells and tissues that results from continuous irritation by a pathogenic agent or environmental factor and leads to development of symptoms. ​[1]​

What is “normal”, or “malfunctioning”? 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” or “continuous irritation”? What is the difference between injury and disease?

Do insects cause disease? According to these definitions, yes, they can, as many are persistent and their interaction with plants otherwise fits definitions of disease. But insect-caused damage is generally excluded from the concept of disease and pathogens. Nematodes are traditionally included. This is just a result of what branch of science traditionally deals with these things.

Do genetic defects and nutrient imbalances fit the definition, as they do in human medicine? For these cases, you may need to add to the first definition the phrase, “or condition.” The second definition includes “or environmental factor”.

Although 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. Why? 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.  This may seem obvious to you.  That’s good; it’s not obvious to everyone.  Spread the word.

In some versions of the disease triangle, the vertices of the triangle represent the three components. In this version, the sides do, and the length of each side represents the relative favorability of that factor for disease. Conceptualy, that contributes to the triangle area, which represents the overall amount or severity of disease.


A pathogen is an agent that causes disease (“path”-“gen”, literally disease-generator). 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 well to deficiencies. Here are the main kinds of biotic pathogens:

  • virus/viroid
  • phytoplasmas
  • bacteria
  • nematodes
  • fungi
  • flowering plants

Signs and symptoms

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

  • White trunk rot by Phellinus igniarius: conk
  • Armillaria root rot: mushroom, rhizomorph, mycelial fan
  • Laminated root rot: conk of Phellinus weirii, setal hyphae

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:

  • Root rots: crown thinning, dieback, resinosis etc. Also decay.
  • Foliage diseases: discolorations, lesions, defoliation
  1. 1.
    Agrios GN. 1997. Plant Pathology. San Diego: Academic Press. 4th ed.
  2. 2.
    Manion PD. 1991. Tree Disease Concepts. Englewood Cliffs, New Jersey: Prentice-Hall. 2nd ed.