Decline of Established Alfalfa in Soils Naturally Infested with Phytophthora megasperma f. sp. medicaginis and Level of Correlation by Seedling Assay. M. J. Havey, Department of Plant Pathology, University of Wisconsin, Madison 53706. C. R. Grau, Department of Plant Pathology, University of Wisconsin, Madison 53706. Plant Dis. 69:221-224. Accepted for publication 31 August 1984. Copyright 1985 The American Phytopathological Society. DOI: 10.1094/PD-69-221.

Decline of established alfalfa stands, as measured by changes in mean disease severity index, number of plants per square meter, and total root and crown dry weight per square meter was studied over 3 yr for 10 commercial cultivars varying in levels of resistance to Phytophthora megasperma f. sp. medicaginis (Pmm). Significant differences between cultivars were found for the three variables in analyses over 3 yr. Significant differences between cultivars were not found after the establishment year for analyses within years. A postemergence damping-off (PEDO) seedling assay was conducted for each of the 10 cultivars and two Pmm isolates. The proportion of seedling survivors after 11 days in the PEDO test correlated significantly with the results of the Minnesota Phytophthora root rot evaluations (0.94 and 0.95 for the two Pmm isolates) and the number of surviving plants per square meter after 3 yr in the field (0.73 and 0.87). The PEDO test was an accurate predictor of the relative rate of decline of alfalfa cultivars under conditions of natural Pmm infestation.