What is a Tree Disease

Forest pathology is the study of tree diseases.

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

  • Any deviation in the normal functioning of a plant caused by some type of persistent agent ​[10]​.
  • 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 ​[2]​.
  • Sustained physiological and resulting structural disturbances of living tissues and organs, ending sometimes in death ​[6]​ (citing ​[7]​).

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”, “continuous irritation”, or “sustained disturbance”? What is the difference between injury and disease?

Do insects cause disease? According to all 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”, and the third doesn’t include cause at all.

When a fungus decays only the heartwood (which has no living tissues), is that a disease? Most of us would think so. The first two definitions seem to include that, but the third doesn’t. (However, on p. 11 of his landmark textook ​[6]​, Boyce clearly includes that example under disease.)

The disease triangle

The disease triangle is a useful, time-tested conceptual model for disease. It is taught in most introductory courses and commonly used conceptually by pathologists. Why? Because:

  • It emphasizes that disease is not just caused by a pathogen, but by the interaction of the environment, a pathogen, and a host (suscept) to produce disease.
  • It applies also to diseases where environment is especially important or multiple pathogens are involved (sometimes called ‘declines’).
  • It 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. Conceptually, that affects 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 generally refers to living agents. Here are the main kinds of pathogens:

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

Although most pathogens are parasites, it is technically possible for a non-parasite to cause disease. Consider a vine that grows over the crown of a tree, blocking light and killing it. That fits definitions of disease. Imagine an organism in the soil that does not infect but releases a toxin that damages roots. So it’s possible.

It is also possible, and more common, for a parasite to be non-pathogenic:

  • Many fungi live as endophytes inside tree organs such as leaves or xylem, e.g. ​[5, 9]​. Although they derive their nutrition from the plant, they cause no perceptible damage, and in some cases are beneficial, e.g. ​[3, 4, 8]​.
  • Mycorrhizae are rootlets that are parasitized by fungi that cause no damage and provide far more benefit than the cost of feeding the fungi. They are almost universal among plants, including trees.
  • In addition, many pathogens have an extended phase after infecting a plant, during which they cause no damage, e.g. ​[1, 11, 12]​. These are called latent infections. Disease expression may depend on triggers such as host stress.

Signs and symptoms

Plant pathologists make a distinction in manifestations of disease: Signs are physical appearances of of a pathogen, either somatic tissues or fruiting. Boyce followed Whetzel in including “exudations” (slime flux, gummosis, resinosis) under signs ​[6]​, but that concept is not used today. 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
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  2. 2.
    Agrios GN. 1997. Plant Pathology. San Diego: Academic Press. 4th ed.
  3. 3.
    Bier JE. 1964. The relation of some bark factors to canker susceptibility. Phytopathology 54(3):250–253.
  4. 4.
    Bier JE. 1966. The possibility of microbiological types with different degrees of disease resistance within a tree species or clone. In: Breeding Pest-Resistant Trees, pp. 257–270. Elsevier <https://linkinghub.elsevier.com/retrieve/pii/B9780080117645500547>.
  5. 5.
    Bills GF, Redlin SC, Carris LM. 1996. Isolation and analysis of endophytic fungal communities from woody plants. In: Endophytic Fungi in Grasses and Woody Plants: Systematics, Ecology, and Evolution, pp. 31–65. St. Paul, Minnesota, USA: APS Press.
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    Boyce JS. 1961. Forest Pathology. The American Forestry Series vol. New York: McGraw-Hill Book Company, Inc. 3rd ed.
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    Hata K, Futai K. 1996. Variation in fungal endophyte populations in needles of the genus Pinus. Canadian Journal of Botany 74:103–114.
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    Manion PD. 1991. Tree Disease Concepts. Englewood Cliffs, New Jersey: Prentice-Hall. 2nd ed.
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    Nugent LK, Sihanonth P, Thienhirun S, Whalley AJS. 2005. Biscogniauxia: a genus of latent invaders. Mycologist 19(1):40–43 <https://www.sciencedirect.com/science/article/abs/pii/S0269915X05001060>.
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