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Specificity of Resistance to Aphanomyces euteiches in Seedling Alfalfa. E. B. Holub, Former Graduate Research Assistant, Department of Plant Pathology, University of Wisconsin, Madison 53706. C. R. Grau, Professor, Department of Plant Pathology, University of Wisconsin, Madison 53706. Plant Dis. 74:164-168. Accepted for publication 19 July 1989. Copyright 1990 The American Phytopathological Society. DOI: 10.1094/PD-74-0164.

A bioassay was used to evaluate the reaction of alfalfa seedlings to Aphanomyces euteiches. Four-day-old seedlings were inoculated with zoospores, and disease severity was rated 10 days later on a five-class scale where 1 = healthy plant and 5 = dead plant. Plants were selected within classes, self-pollinated, and evaluated by progeny testing for reactions to A. euteiches. After progeny tests, 33 of 34 class 2 parent plants were rated as resistant to A. euteiches, compared with three of four class 3 parents and none of four class 4 parents. The parents were derived from alfalfa cultivars that yielded no class 1 plants when assayed against A. euteiches. When class 2 and class 3 plants were selected from these cultivars and self-pollinated, however, 3060% of the progeny were rated as class 1. Experimental populations of alfalfa were created to determine the specificity of resistance to A. euteiches. Resistance to A. euteiches in these populations was not effective against disease caused by Phytophthora megasperma f. sp. medicaginis, and resistance to P. m. f. sp. medicaginis was similarly ineffective against A. euteiches. In another experiment, plants resistant to isolates of A. euteiches recovered from alfalfa were resistant to isolates from peas. However, pea isolates were less virulent than isolates from alfalfa, even on alfalfa populations previously selected against isolates from pea.