The epidemiology of leaf spot disease in a native prairie. I. The progression of disease with time

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Abstract
Leaf spot disease caused principally by Pyrenophora tritici-repentis was studied in 1969, 1970, and 1971 at Matador, Saskatchewan, in a native prairie dominated by Agropyron dasystachyum and A. smithii. The proportion of leaf area diseased (x), the lesion area per square metre of plot area, and the number of lesions per square metre of plot area were all used to measure disease on the total graminoids in the ecosystem. Disease progress curves for the first two variables differed considerably. The maximum x values in 1969, 1970, and 1971 were 0.034, 0.043, and 0.027 respectively. However the maximum lesion area per square metre of plot in 1970 was about twice that of the other 2 years. In 1970 there was less disease in an irrigated treatment and in a treatment in which the grass had been burned in late summer 1969, than in non-treated grassland. In 1970 and 1971 the distribution of individual leaf spots in 13 size classes varied at each sampling date, but most lesions were always less than 0.31 mm2. Overall, disease intensities were always low, despite sometimes apparently favorable environmental conditions. The results are discussed in relation to (inter alia) the study of plant disease epidemics in natural ecosystems and the variables generally used to measure diseases.

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