Decline of Established Alfalfa in Soils Naturally Infested withPhytophthora megaspermaf. sp.medicaginisand Level of Correlation by Seedling Assay

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Abstract
Decline of established alfalfa stands, as measured by changes in mean disease severity index, number of plants/m2, and total root and crown dry weight/m2 was studied over 3 yr for 10 commercial cultivars varying in levels of resistance to P. megasperma f. sp medicaginis (Pmm). 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 2 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 2 Pmm isolates) and the number of surviving plants/m2 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.