STUDIES ON THE EPIDEMIOLOGY OF CROWN BUD ROT OF ALFALFA IN SOUTHERN ALBERTA

Abstract

Crown bud rot is widespread in irrigated alfalfa stands in southern Alberta. Rhizoctonia solani Kühn, Fusarium roseum Link sensu Snyder & Hansen, and Ascochyta imperfecta Peck acting alone or in combination produce brown to black lesions on the crown buds of plants in their second and subsequent years of growth. The host plant is most susceptible to the disease in early spring after winter dormancy. Soil temperatures above 16 °C. appear to reduce disease development. Relative occurrence of species isolated from infected buds indicate a succession of fungi. F. roseum is the predominant isolate until the third year of alfalfa growth when its numbers are approximately equalled by R. solani. The latter then decreases in prevalence possibly because of the antibiotic action of saprophytic species of Trichoderma and Penicillium. F. roseum maintains about the same degree of activity throughout the growing season whereas R. solani generally appears most frequently in summer-sampled alfalfa and A. imperfecta in spring samples.