Reactions in the Annual Medicago spp. Core Germ Plasm Collection to Phoma medicaginis.

The annual Medicago spp. core collection, consisting of 201 accessions, represents the genetic diversity inherent in 3,159 accessions from 36 annual Medicago spp. This germ plasm was evaluated for resistance to spring black stem and leaf spot caused by Phoma medicaginis. Spring black stem and leaf spot is a major destructive disease in perennial alfalfa (Medicago sativa) grown in North America, Europe, and other temperate regions. Disease control is based principally on the use of cultivars with moderate levels of resistance. Evaluation of the core collection was conducted using standardized environmental conditions in growth chambers, and included the M. sativa standard reference cultivars Ramsey (resistant) and Ranger (susceptible). The degree of resistance found among accessions within species was variable, but most annual species and accessions were susceptible. Most accessions from 10 species exhibited high disease resistance. These included accessions of M. constricta, M. doliata, M. heyniana, M. laciniata, M. lesinsii, M. murex, M. orbicularis, M. praecox, M. soleirolii, and M. tenoreana. Most of the accessions within M. arabica, M. minima, M. lanigera, M. rotata, M. rugosa, M. sauvagei, and M. scutellata were highly susceptible. Disease reactions among some accessions within species were highly variable. On a 0-to-5 disease severity scale, ratings ranged from 0.67 (PI 566873) to 4.29 (PI 566883) within accessions of M. polymorpha. Most of the M. truncatula accessions were susceptible, with a mean of 3.74. Resistant reactions were similar to those found in incompatible interactions with P. medicaginis and alfalfa, which have been associated with specific genes leading to the production of isoflavonoid phytoalexins. The large genetic variability in annual Medicago spp. offers potential for locating and utilizing disease resistance genes through breeding or genetic engineering that will enhance the utilization of Medicago spp. as a forage crop.