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 ​[12]​.
  • 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?

Note that all definitions refer to duration of the damage. 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.)


A pathogen is an agent that causes disease (“path”-“gen”, literally disease-generator). It generally refers to living agents, but doesn’t have to. Here are the main kinds of biotic 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, 11]​. Although they derive their nutrition from the plant, they cause no perceptible damage, and in some cases are beneficial, e.g. ​[3, 4, 9]​.
  • There is usually some time between infection and symptom expression, for which we use the term latent infection. Some pathogens have an extended latent period, during which they cause no damage, e.g. ​[1, 13, 15]​. They initially behave as endophytes, but later may cause disease. Disease expression may depend on triggers such as host stress.
  • 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.

The Disease Triangle

The disease triangle is a useful and flexible conceptual model for disease. It is taught in most introductory courses and commonly used conceptually by pathologists. In it, plant disease is seen as the result of interaction of three elements:

  • Host: The species, genetics (cultivar, genes for resistance, etc.), age, physiological status and other characteristics can all influence susceptibility to and progress of disease.
  • Pathogen: Similarly, the pathogen species, virulence genes, inoculum potential, and other characteristics all influence the likelihood and severity of disease. However, abiotic diseases also can be considered with the disease triangle.
  • Environment: The biotic and abiotic environment influence disease in many complex ways. Organisms other than the pathogen, such as endophytes, competitors, antagonists, and other host stress agents can have positive or negative influence on disease. The abiotic environment is also hugely important in almost every type of plant disease.

Why is the disease triangle so useful and widely adopted? 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.
  • 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.

Early authors generally recognized the importance of the three elements in determining disease, but Gäumann was the first to lay out the triad in detail and exhaustively document the importance of each element ​[10]​. Stevens was likely the first to portray the relationship as a triangle ​[8, 14]​. 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.

Signs and symptoms

Plant pathologists make a distinction in manifestations of disease: Signs are physical appearances 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 decayed wood.
  • Foliage diseases: discolorations, lesions, defoliation

Infected vs. infested

What is the difference between “infected” and “infested”? Infected refers to plants or plant parts that a biotic pathogen has entered. Infested refers to areas, stands, plots, soil, containers, etc., where the pathogen is present. Examples: In plot 1, 3 of 10 trees were infected. Plots 1 and 10 were infested. More than 1,000 ha of loblolly pine plantations are infested with brown spot needle blight. Infected needles can be identified by brown spots with yellow halos. Seedlings were infected because the soil they were grown in was infested.

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