STUDIES ON SWEET CLOVER FAILURES IN SOUTHWESTERN ONTARIO

Abstract

The etiology of sweet clover failures that have occurred in recent years in southwestern Ontario was investigated further through studies of the possible role of adverse environmental factors, of insects, and of pathogenic bacteria and fungi. A number of soil-borne parasites were found. They exhibited unusual parasitism at the point of initial infection and as the disease occurred in the field. Phytophthora cactorum was found to cause the disease, and its parasitism of sweet clover was investigated. The susceptibility of the host varied sharply with its stage of development and was at a minimum in the seedling stage and greatest early in the second season's growth. Numerous lenticels on the tap roots provided natural breaks in the periderm through which the fungus probably entered. Invasion of the tissue by the fungus was intracellular in the relatively small number of instances when first season plants were attacked and was always intercellular when second season plants were attacked. The fungus sporulated when seedlings were infected and when extracts of seedlings were added to common artificial media. Mycelium of the fungus was found abundant in cortical parenchyma considerably in advance of lesions. Secondary organisms quickly followed the initial infection. Epidemiological studies of sweet clover failures indicated that outbreaks were characterized by relatively short periods of high disease incidence in the early part of the second growing season in individual clover stands. An analysis of weather data for the period during which recent epidemics occurred suggested that sustained high soil moisture in the early part of the growing season was prerequisite to epidemic outbreaks of the disease, with higher than average soil temperatures favorable as well. No phonological condition, however, seemed responsible for confining the outbreaks to the beginning of the second season of growth, which appeared to be incidental to some physiological change correlated with the developmental rhythm of the host.