Cotyledons of 10-day-old alfalfa seedlings exhibited susceptible and resistant disease reactions within 3-7 days after inoculation with zoospores of Phytophthora megasperma.  Susceptible reactions, characterized by rapid collapse of cotyledon tissue leading to shriveling and complete necrosis, contrasted with resistant reactions, in which only small, red-brown local lesions developed on upper surfaces of infected cotyledons.  Symptoms on some plants were intermediate between resistant and susceptible types.  Differential disease symptoms were also observed in detached leaflets inoculated with zoospores.  Similar numbers of Vernal alfalfa seedlings were severely diseased at inoculum levels of from 9 to 151 zoospores per cotyledon; fewer plants were severely diseased when fewer than five zoospores were applied to each cotyledon.

Cotyledons of Vernal and Saranac were more susceptible to infection by zoospores than were cotyledons of six other lines and cultivars which had been selected for resistance to Phytophthora root rot.  Vernal and Saranac plants also developed more severe root rot in artificially infested soil than did plants of the other six lines.  The significant correlations observed between resistance and susceptibility in cotyledons and tap- and lateral roots of alfalfa (r = .80 and r = .78, d6 df) suggest that the cotyledon inoculation technique may provide a simple means for screening alfalfa lines for resistance to P. megasperma.

Additional key words: Medicago sativa.