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Development of Foliar Diseases of Alfalfa in Relation to Microclimate, Host Growth, and Fertility. K. M. Emery, Graduate research assistant, Department of Plant Pathology, University of Missouri, Columbia 65211; J. T. English, assistant professor, Department of Plant Pathology, University of Missouri, Columbia 65211. Phytopathology 84:1263-1269. Accepted for publication 1 September 1994. Copyright 1994 The American Phytopathological Society. DOI: 10.1094/Phyto-84-1263.

Foliar diseases of alfalfa in four fertility treatments and an untreated control were evaluated in relation to microclimate, host growth, and soil fertility during 1991 and 1992. Over the course of six growth periods, alfalfa growth and disease development varied significantly. Disease incidence and severity were greater on leaves from the lower half of shoots than on leaves from the upper half. Disease development depended on moisture conditions. The relationship of disease to moisture varied between the upper and lower halves of shoots. Disease on the lower half of shoots was correlated with leaf wetness duration as well as atmospheric moisture conditions; disease on the upper half of shoots was correlated only with cumulative rainfall. Although disease incidence and severity were not correlated significantly with alfalfa growth or soil fertility, growth did influence disease assessments by altering the composition of the pool of leaves sampled. As shoot growth proceeded, newly produced, uninfected leaves reduced the proportion of diseased leaves in the sampling pool, thereby reducing disease incidence and severity.