STUDIES ON THE WILT OF PEAS CAUSED BY FUSARIUM OXYSPORUM SCHL. F. PISI (LINF.) S. & H., RACE

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Abstract
Fusarium oxysporum Schl. f. pisi (Linf.) S. & H., race 1, was found associated with pea wilt in two localities on the Niagara Peninsula. Under experimental conditions, low soil temperature was found to be the primary limiting factor to the development of this disease. Secondary factors governing disease severity included the nature and concentration of pathogenic inoculum, the microbial population of the soil, and the particular host variety infected. Two pathogenic isolates of race 1 could not be distinguished in vitro with respect to morphological and physiological characters examined.A stable cultural variant isolated from a sectored region of a wild-type colony was found to differ from the parental strain in colonial characters, pigmentation, rate of growth and sporulation, pH drifts of the substrate medium incident to the growth of the fungus, sensitivity to the antifungal antibiotic Acti-dione, and inactivation by ultraviolet irradiation. The variant strain was also less virulent on host varieties susceptible to the wild-type race 1 isolate.Auxotrophic mutants were induced by ultraviolet irradiation and were used in the synthesis of heterocaryons. A diploid strain was screened from one such induced heterocaryon. Positive evidence for the heterocaryotic and diploid stages of the parasexual cycle in this pathogen is presented.