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 temperate regions of the world, but little is known about its host range within the annual Medicago species (medics). Several medics have characteristics potentially beneficial to sustainable agriculture programs and to the genetic improvement of alfalfa. Therefore, we evaluated resistance in 199 accessions representing 33 species of the annual Medicago core germ plasm collection to P. trifoliorum isolates I7 and I8 from alfalfa from Kansas and southern California, respectively. Seedlings at the cotyledonary growth stage were inoculated with conidia suspended in water. One week later, the plants were rated resistant (no conidium production) or susceptible (conidium production). 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 hypersensitive reactions on the inoculated cotyledons. All but one of the medic accessions had a higher percentage of plants resistant to both isolates than did cv. Saranac alfalfa, the resistant control. Conidium production generally was much less intense on the susceptible medic plants than on the alfalfa controls. A greater percentage of plants in four medic accessions was more resistant to I8 than to I7, and 22 were more resistant to I7 than to I8.