Reactions of Accessions in the Annual Medicago Core Germ Plasm Collection to Erysiphe pisi

Reaction to Erysiphe pisi was evaluated in the annual Medicago core germ plasm collection of 199 USDA Plant Introduction accessions from 33 species. Seedlings at the first trifoliolate leaf stage were inoculated with conidia and maintained in growth chambers for 21 days at 20°C, 70 to 90% relative humidity, and a 12-h photoperiod. Beginning 4 days after inoculation, plants with the first sign of powdery mildew were recorded and removed daily. Results for each accession were presented as the percentage of plants mildew-free 21 days after inoculation and also as area under the disease progress curve (AUDPC) values expressed as standardized proportion/days. AUDPC values were useful to account for length of latent period (number of days from inoculation until first sign of mildew). Thirty-two accessions from 14 species included plants mildew-free 21 days after inoculation. Mildew-free plants in those 32 accessions ranged from 100% in M. heyniana PI 537136 and M. truncatula PI 292436 down to 1% in six accessions. All plants in 87 accessions, which included all accessions of 11 species, had powdery mildew 4 days after inoculation and thus expressed no resistance. The remaining 80 accessions included slow-mildewing plants with latent periods of 5 to 14 days. This slow-mildewing trait may be a useful source of resistance to E. pisi.