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Formae Speciales Differentiation of Phytophthora megasperma Isolates From Soybean and Alfalfa. Ta -Li Kuan, Department of Plant Pathology, University of California, Riverside, 92521, Senior author is now at Asgrow Seed Co., 500 Lucy Brown Lane, P.O. Box L, San Juan Bautista, CA 95045; D. C. Erwin, Department of Plant Pathology, University of California, Riverside 92521. Phytopathology 70:333-338. Accepted for publication 10 October 1979. Copyright 1980 The American Phytopathological Society. DOI: 10.1094/Phyto-70-333.

Colony morphology of thirty isolates of Phytophthora megasperma from various sources varied with isolates and growth media. There were no distinct differences in oospore morphology and the diameters of oogonia ranged from 30 μm to 58 μm and appeared to be of a single continuously varying population. Isolates from alfalfa and soybean were pathogenic only to the hosts from which they were isolated. Special forms were proposed to subdivide the alfalfa and soybean isolates of P. megasperma into Phytophthora megasperma f. sp. medicaginis for isolates that attack alfalfa and Phytophthora megasperma f. sp. glycinea for isolates that infect soybean.