APHANOMYCES-EUTEICHES IN ALFALFA FIELDS OF QUEBEC - ISOLATION, MORPHOLOGY AND VARIABILITY OF ITS GROWTH AND PATHOGENICITY

Sixteen strains of Aphanomyces euteiches were obtained from alfalfa seedlings by a baiting technique. They were found in the soil of 10 out of the 12 alfalfa fields sampled. In spite of some morphological variability, these strains were not taxonomically distinguishable from one another. They had different cultural characteristics (minimal, maximal, and optimal growth temperatures). Their pathogenicity was tested on alfalfa, garden pea, green bean, and garden beet. The green bean was lightly affected and a wide spectrum of pathogenicity was observed on alfalfa. Four pathogenicity groups were distinguished: (i) 13% of the strains were nonvirulent to the four hosts, (ii) 25% were specific to alfalfa, (iii) 25% were more virulent to alfalfa than to garden pea, and (iv) 37% were slightly pathogenic to both alfalfa and garden beet. The last-group was characterized by a reduced mycelial growth at temperatures above 24-degrees-C and the symptoms noticed on garden beet were similar to those caused by Aphanomyces cochlioides.