Sources of resistance to anthracnose in the annual Medicago core collection

The annual genus Medicago core collection, consisting of 201 accessions, re presents the genetic diversity inherent in 3,159 accessions from 36 annual Medicago species. This germ plasm was evaluated for resistance to anthracno se caused by Colletotrichum trifolii. Anthracnose is a major disease in per ennial alfalfa (Medicago sativa L.) grown in North America and disease cont rol is based principally on the use of resistant varieties. Evaluation of t he core collection was conducted using standardized environmental condition s in growth chambers, and included the M. sativa standard reference cvs. Ar e (resistant) and Saranac (susceptible). The degree of resistance found amo ng accessions within species was highly variable; however, most annual spec ies and accessions were susceptible. Only 14 accessions from seven species exhibited resistance greater than 40% seedling survival. These included acc essions of M. murex, M. muricoleptis, M. polymorpha var. brevispina, M. pol ymorpha var. polymorpha, M. radiata, M. soleirolii, M. truncatula, and M. t urbinata. Of the 12 accessions of M. polymorpha var, polymorpha, 4 exhibite d more than 50% resistance, but 3 accessions were 100% susceptible. Most of the M. truncatula and M. turbinata accessions exhibited significantly more resistance than accessions of other species. Plant introduction (PI) acces sion number PI 495401 of M. muricoleptis exhibited 90.3% resistance. Access ions of M. scutellata were uniformly susceptible. Histological examinations of 14 of the most anthracnose-resistant accessions revealed that C. trifol ii spores germinated and produced typical appressoria, but failed to penetr ate and produce the primary and secondary hyphae characteristic of suscepti ble interactions. Resistant reactions were similar to those found in incomp atible interactions with C. trifolii and alfalfa, which have been associate d with specific genes leading to the production of isoflavonoid phytoalexin s. The large genetic variability in annual Medicago spp. offers potential f or locating and utilizing disease resistance genes through breeding or gene tic engineering that will enhance the utilization of Medicago spp. as a for age crop.