Reactions in the annual Medicago core germ plasm collection to two isolates of Peronospora trifoliorum from alfalfa

Peronospora trifoliorum causes downy mildew of alfalfa throughout the tempe rate regions of the world, but little is known about its host range within the annual Medicago species (medics). Several medics have characteristics p otentially beneficial to sustainable agriculture programs and to the geneti c improvement of alfalfa. Therefore, we evaluated resistance in 199 accessi ons representing 33 species of the annual Medicago core germ plasm collecti on to P. trifoliorum isolates 17 and 18 from alfalfa from Kansas and southe rn California, respectively. Seedlings at the cotyledonary growth stage wer e inoculated with conidia suspended in water. One week later, the plants we re rated resistant (no conidium production) or susceptible (conidium produc tion). High levels of resistance existed in all species. All plants in 142 (71%) of the accessions were resistant to both isolates, but plants in only 24 of those resistant accessions were free of chlorotic to necrotic hypers ensitive reactions on the inoculated cotyledons. All but one of the medic a ccessions had a higher percentage of plants resistant to both isolates than did cv. Saranac alfalfa, the resistant control. Conidium production genera lly was much less intense on the susceptible medic plants than on the alfal fa controls. A greater percentage of plants in four medic accessions was mo re resistant to 18 than to 17, and 22 were more resistant to 17 than to 18.