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Phytophthora Root Rot of Alfalfa in Central New York. H. T. Wilkinson, Research Assistant, Department of Plant Pathology, Cornell University, Ithaca, NY 14850. R. L. Millar, Professor, Department of Plant Pathology, Cornell University, Ithaca, NY 14850. Plant Dis. 65:127-129. Copyright 1981 American Phytopathological Society. DOI: 10.1094/PD-65-127.

Nine counties in New York’s Erie-Ontario Plain and Mohawk Valley were surveyed for Phytophthora root rot of alfalfa, caused by Phytophthora megasperma. Diseased plants were observed in 66% of alfalfa fields that had been saturated for a week or more. First-year stands were the most severely attacked, but Phytophthora root rot appeared to be an important factor in alfalfa stand decline regardless of the age of the stand. Most severe root rot was associated with sandy clay loam soils with fair to moderately poor drainage. Phytophthora activity in soil, as detected by a seedling baiting assay, was positively correlated with the severity of root rot in the field. P. megasperma was isolated from 82% of the fields seeded with alfalfa and 71% of the fields seeded to other crops or left fallow. Resistant varieties and tile drainage did not appear to be satisfactory solutions for control.