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Variability in Virulence of Aphanomyces euteiches. C. R. Grau, Department of Plant Pathology, U.S. Dairy Forage Research Center, University of Wisconsin, Madison 53706. A. M. Muehlchen, J. E. Tofte, and R. R. Smith. Department of Plant Pathology; and Agricultural Research Service, U.S. Department of Agriculture, U.S. Dairy Forage Research Center, University of Wisconsin, Madison 53706. Plant Dis. 75:1153-1156. Accepted for publication 7 May 1991. Copyright 1991 The American Phytopathological Society. DOI: 10.1094/PD-75-1153.

Host specificity of Aphanomyces euteiches was studied with 14 isolates from throughout the United States and Canada. Isolates were obtained by baiting from soil samples and by direct recovery from roots of field-grown plants. At least one of the 14 isolates of A. euteiches was pathogenic to alfalfa; red, berseem, and crimson clovers; white and yellow sweet-clovers; field pea; garden pea; snap bean; lima bean; and faba bean. Isolates of A. euteiches differed in the expression of virulence on specific hosts. None of the isolates were pathogenic to subterranean, arrowleaf, alsike, or ladino clovers; crown vetch; birdís-foot trefoil; soybean; cowpea; lupine; or peanut. The isolate hosts ranged from multiple hosts (isolate from pea) to host specific (isolate from snap bean). Nine of 27 isolates of A. euteiches were highly virulent on an alfalfa population (WAPH-1) selected for resistance to isolates MD433, MN122, and NY101. Isolates were recovered from alfalfa seedlings grown in soils that originated from North Carolina, Mississippi, Virginia, Idaho, Minnesota, and Wisconsin. An alfalfa accession, PI 468051, expressed a disease reaction intermediate between that of a susceptible alfalfa population (Saranac) and WAPH-1 for three isolates and lower than that of WAPH-1 for one isolate. This report provides evidence that A. euteiches is composed of host-specific subpopulations. In addition, two alfalfa populations reacted differentially to isolates of A. euteiches.