What is a Tree Disease

Forest pathology is the study of tree diseases.

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.”

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.  Spread the word.

In concept, 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.


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 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