DEVELOPMENT OF FOLIAR DISEASES OF ALFALFA IN RELATION TO MICROCLIMATE, HOST GROWTH, AND FERTILITY

Foliar diseases of alfalfa in four fertility treatments and an untreat ed control were evaluated in relation to microclimate, host growth, an d soil fertility during 1991 and 1992. Over the course of six growth p eriods, alfalfa growth and disease development varied significantly. D isease incidence and severity were greater on leaves from the lower ha lf of shoots than on leaves from the upper half. Disease development d epended on moisture conditions. The relationship of disease to moistur e 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 shoo ts was correlated only with cumulative rainfall. Although disease inci dence and severity were not correlated significantly with alfalfa grow th or soil fertility, growth did influence disease assessments by alte ring the composition of the pod of leaves sampled. As shoot growth pro ceeded, newly produced, uninfected leaves reduced the proportion of di seased leaves in the sampling pool, thereby reducing disease incidence and severity.