STUDIES ON ASCOCHYTA IMPERFECTA PECK: PARASITIC STRAINS AMONG FIFTY ISOLATES FROM CANADIAN ALFALFA SEED

Abstract

Fifty isolates of Ascochyta imperfecta from alfalfa seed grown in Ontario, Manitoba, Saskatchewan, Alberta, and British Columbia produced varying degrees of infection on excised leaves of Medicago falcata, Medicago sativa (three strains), Melilotus alba, Melilotus officinalis, Trifolium pratense, Lotus corniculatus, Pisum sativum, and Vicia faba. F tests from analyses of variance of disease ratings showed that the differences in susceptibility between hosts and between strains of Medicago were highly significant. Among the 10 hosts and within the strains of Medicago, the interaction isolates × hosts was highly significant. Thus, specificity among the isolates was clearly demonstrated. This is interpreted to mean that parasitic strains existed among the 50 isolates.A. imperfecta was isolated from 50 to 70% of the seed samples from each province. The heaviest infestation was on seed from Manitoba, where the weather at harvest time was wet and where the crop overwintered in the field.