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Plant disease epidemiology? How can there be an epidemic on plants? (Gr. epi, on; demos, people) Well, strictly speaking, we should call it an epiphytotic, but the term "epidemic" has become so widely used in reference to plant disease and so entrenched in the phytopathological literature that we are forced to give in and leave "epiphytotic" to the etymological purists. So if we accept "epidemic" to apply to plant populations, then "epidemiology" refers to the study of the development of disease in those populations. However, these terms still are technically imprecise and sometimes confusing because of their common usage. "Epidemic" is often used in such expressions as "epidemic proportions" to mean a high level of disease. "Epidemic" also is commonly used to denote a sudden and rapid or a widespread development of disease, implying that if development is slow or it is spatially limited there is no epidemic.

The Dynamics of Plant Disease

In this text we will use "epidemic" to denote the dynamics of disease, that is, the change in the amount of disease with time. By "amount" of disease we mean either incidence (the number of plants or plant parts affected), severity (the amount of tissue affected), or a combination of both. By this definition there can be fast epidemics or slow epidemics, and there can even be "negative" epidemics, in which there is a decrease in the amount of disease with time. For example, if we are using the proportion of leaves infected as a measure of disease incidence, and during a dry spell we get new, healthy leaves forming while the infected ones drop off, we could get a negative change in the amount of disease.

The Spatial Distribution of Plant Disease After a period of time, a disease that started with a single infected plant will commonly appear as a "focus", with the density of lesions or of infected plants highest in the center and diminishing radially outward.

Associated with the change in the amount of plant disease with time is a change in the spatial distribution of lesions or of infected plants. A good working definition of epidemic, therefore, is a change in plant disease in time and space.

The term endemic is sometimes used (erroneously) to mean the opposite of "epidemic", that is, to denote constant disease, because in areas where the pathogen and its host have coevolved over long periods of time, they often have reached an equilibrium where there is little change in the amount of disease with time. In the strict sense "endemic" means native to a particular area, and therefore we can have epidemics of endemic diseases.