STUDIES ON THE PARASITISM AND VARIATION OF ALTERNARIA RAPHANI

Abstract

The natural infection of radish seed with A. Raphani may result in a lack of germination, a pre- or postemergence blight, a distinctive lesioning of cotyledons and hypocotyls, the presence of scablike lesions on table radish, and in the spotting and blighting of leaves, stalks, and siliques. The fungus was isolated from the internal tissues of all parts of dormant radish seed. Although the pathogen has been reported only on radish in Canada and the United States, the present investigation shows that Canadian isolates are capable of causing a severe leaf blight of stocks and wallflowers. Under field conditions at St. Catharines, Ont., most rapid progress of the disease occurred at temperatures within the optimum range for the fungus, i.e., 22° to 26 °C. Experimental evidence suggests that A. Raphani does not establish an overwintering inoculum in the soil by means of diseased plant debris. Increased soil moisture was associated with increased seedling disease. At a high soil moisture content, infection was lowest at 18 °C.; at medium soil moisture, it was lowest at 18 °C. and also at 23 °C., the next highest experimental temperature.Monosporous isolations showed the presence of numerous wild type strains of A. Raphani which were closely related culturally. Five of these studied intensively differed widely in virulence and sporulation, but had similar growth rates and nutritional requirements for maximum growth. Although most isolates of A. Raphani produced only a few spores in ordinary agar cultures, abundant sporulation was obtained by wounding plate cultures and removing the lids of the culture plates. In agar culture, the wild types readily produced mostly appressed variant strains also showing close cultural relations. These variants exhibited wide differences in pathogenicity, rate of growth, and nutritional requirements, but all showed practically complete loss of sporulation either in normal or wounded cultures. The effects of cultural variation of wild type strains on cultural habit, pathogenicity, rate of growth, sporulating capacity, and nutritional requirements were random and unrelated. These data, as well as the spontaneous origin and irreversibility of the variants, favored the view that they arose in culture by mutation in the naturally occurring strains.A. Raphani was shown to be capable of surviving at least 18 months in dry soil cultures with no loss of cultural habit, virulence, or sporulation.Appreciable increase in emergence and decrease in seedling infection was obtained by seed treatments with some of the common fungicidal dusts.