Disease Progress

Disease Progress

Disease on plants usually starts out at a low level, a small number of plants affected and a small amount of plant tissue affected, and it becomes of concern to us only when its incidence and severity increases with time. When we look at some examples of plant disease epidemics from the published literature, we not only notice that the incidence or severity starts near zero and then increases dramatically, but we also can discern some distinct patterns of development with time. For example, in Phytophthora blight of pepper seedlings (Phytophthora capsici) and Fusarium kernel rot (Fusarium moniliforme, currently F. verticilloides) of maize, disease progress is roughly linear (allowing for some minor deviations that we can consider random error).

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Phytophthora blight of pepper seedlings	Fusarium kernel rot of maize

On the other hand, in bean rust (Uromyces phaseoli) and grey leaf spot of corn (Cercospora zeae-maydis), there is a definite upward curve; that is, disease increases at an increasing rate, a curve we could call exponential.

Bean rust Grey leafspot of maize

Obviously plant disease cannot continue to increase forever, and as the level of disease approaches 100%, the disease progress curve gradually flattens out. For example, in epidemics such as the infection of beans by Sclerotium rolfsii or the infection of tobacco by Phytophthora nicotianae, disease progress starts out looking linear but slows down as it approaches a maximum.

Sclerotium rolfsii on beans Black shank on tobacco

Likewise, the disease progress curves of Puccinia graminis subsp. graminicola on ryegrass and Pyrenophora teres f. sp. teres on barley appear exponential at first, but as time goes on and the incidence and severity of disease approach 100%, the rate of disease progress gradually slows to zero, giving both curves a somewhat sigmoid shape ("S" shape).

Black stem rust on ryegrass Net blotch on barley

To be sure, not all examples of disease progress can be as neatly categorized as these, but in general plant disease epidemics tend to be either roughly linear or exponential in the early stages, and they tend to level off as they approach some limit.

The impact of plant disease and the losses that it causes are a function of disease progress. To reduce this impact, we need not eliminate the disease, we merely need to keep disease development below an acceptable level. That means that the progress of disease and the factors that influence disease progress must be understood in quantitative terms. We have to know what kinds of diseases lead to linear disease progress and what factors affect the slope of the line (the rate of disease progress). Likewise, we have to know what kinds of diseases tend to produce exponential disease progress curves and how we can reduce both the starting level of disease and the rate of epidemic development. Finally, we have to know why epidemics sometimes level off and what imposes limits to their development.

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