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Ecology and Epidemiology

Comparative Virulence of Aphanomyces euteiches f. sp. phaseoli and Pythium ultimum on Phaseolus vulgaris at Naturally Occurring Inoculum Levels. W. F. Pfender, Research associate, Department of Plant Pathology, University of Wisconsin, Madison 53706; D. J. Hagedorn, professor, Department of Plant Pathology, University of Wisconsin, Madison 53706. Phytopathology 72:1200-1204. Accepted for publication 17 February 1982. Copyright 1982 The American Phytopathological Society. DOI: 10.1094/Phyto-72-1200.

Aphanomyces euteiches f. sp. phaseoli was found to occur commonly in the sandy soil of Wisconsin's major snap bean production area. In greenhouse testing of soil inoculum levels typical for the area, this pathogen caused more damage to beans than did Pythium ultimum at 24 and 28 C. The two pathogens were equally damaging to beans at 20 C, and at 16 C P. ultimum caused more severe disease than did A. euteiches f. sp. phaseoli. When plants were infected by both pathogens simultaneously, disease severity was similar at all temperatures tested. Plants were seldom killed by either pathogen alone, but 3040% of the plants grown at 20 or 24 C were killed if simultaneously infected by both pathogens.