VIEW ARTICLE

Etiology

Morphological Differentiation of Host-Specialized Groups of Phytophthora megasperma. E. M. Hansen, Associate professor, Department of Botany and Plant Pathology, Oregon State University, Corvallis 97331; P. B. Hamm, research assistant, Department of Botany and Plant Pathology, Oregon State University, Corvallis 97331. Phytopathology 73:129-134. Accepted for publication 26 June 1982. Copyright 1983 The American Phytopathological Society. DOI: 10.1094/Phyto-73-129.

Pathogenicity, morphology, and growth of 54 isolates of Phytophthora megasperma from Douglas-fir, soybean, alfalfa, clover, and 10 other hosts were compared. Soybean and clover isolates were highly aggressive only on the host of origin. Douglas-fir and alfalfa isolates formed three pathogenicity groups: D1, aggressive on Douglas-fir; AL1, aggressive on alfalfa; and D2-AL2, weakly aggressive on both hosts. D1 isolates caused a severe reaction in soybean after hypocotyl inoculation, but caused only slight root rot after soil infestation. Isolates from nine other hosts did not cause disease on alfalfa, Douglas-fir, soybean, or clover; an isolate from noble fir was similar to D2-AL2 isolates. Isolates grouped by pathogenicity had similar morphology, growth at several temperatures, and sensitivity to the fungicide metalaxyl. No single morphological character, of the 10 compared, differentiated all host groups, but isolates D1, AL1, and D2-AL2 and those from soybean and clover were separated from each other by combinations of oogonium diameter, sporangium length, and temperature response. Most isolates from hosts other than Douglas-fir, soybean, clover, and alfalfa were similar in morphology and growth to D2-AL2 isolates. The host groups could not be assigned to P. megasperma var. megasperma or var. sojae because many isolates had oogonia of intermediate diameters.