OVERWINTERING OF CERTAIN CEREAL PATHOGENS IN ALBERTA

Abstract

Helminthosporium sativum, Fusarium culmorum, Ophiobolus graminis, Leptosphaeria herpotrichoides, Wojnowicia graminis, Erysiphe graminis, Tilletia caries, and Tilletia foetens readily overwinter under natural conditions at Edmonton, Alberta, Canada. The first five of these overwinter at Edmonton in both spore and vegetative stages and are highly resistant to cold. Even in a non-hardened condition several of them survived severe frost. Young germ tubes of H. sativum for instance continued growth after being frozen solid overnight. Fresh agar cultures of H. sativum, F. culmorum and O. graminis grew vigorously after exposure to sub-zero temperatures. Agar cultures of H. sativum and F. culmorum were viable after a 17-day exposure to temperatures ranging from about 0° F. to —50° F.Conidia of H. sativum proved less resistant to freezing and thawing than to continuous freezing. They survived longer than conidia of F. culmorum and F. graminearum. Mycelia of all foot-rot fungi grown on sterilized barley seeds were viable in one case after three months of continuous freezing, and in another after 40 alternate freezings and thawings. H. sativum and F. culmorum growing in soil survived 61 alternate freezings and thawings.H. sativum, F. culmorum and L. herpotrichoides, retained their viability more readily on the soil surface than when buried at depths of from 2 to 12 in. Well aerated soil seemed to favor the survival of H. sativum, although other factors besides aeration probably are involved. Strains of H. sativum from high latitudes were not better adapted to low temperatures than strains from lower latitudes.The bunt fungi, T. caries and T. foetens, are shown to be capable of overwintering at Edmonton in the form of mycelia in winter wheat. Infection of winter wheat from soil-borne spores may occur in western Canada, but in these experiments soil-borne spores did not survive to infect wheat in the spring.Erysiphe graminis overwinters in the perithecial stage at Edmonton. In the studies made, ascospores were differentiated in the spring, when favorable conditions prevailed and before the first infections of winter wheat were observed.