In soil tube culture, representatives of three biologic forms of Fusarium oxysporum survived unchanged morphologically for 11 years or more. An isolate of the muskmelon wilt fungus remained viable after 17 years' storage in dry air at a temperature of from 3 degrees to 4 degrees C. The surviving unit was found to be the chlamydospore.
Studies conducted in steam-sterilized soil inoculated with Verticillium alboatrum and Pratylenchus penetrans demonstrated a synergistic relationship between fungus and nematode. At low and intermediate levels of Verticillium inoculum, the nematode increased wilt and the increase may be a function of the number of nematodes present. Except at the high inoculum levels of Verticillium, significantly larger numbers of P. penetrans occurred in eggplant roots in the presence than in the absence of the fungus. The nematode alone had no adverse effect on root condition or growth of eggplant.
An epidemic of Verticillium wilt, which occurred in the Niagara Peninsula in 1940, involved many hosts, from seven of which the fungus was isolated; the isolates were compared with respect to cultural characters and pathogenicity. Although the isolates varied slightly in morphology and pathogenicity, they are all referred to Verticillium albo-atrum R. & B.A considerable residuum of inoculum was demonstrated in the soil in May, 1941, following the 1940 epidemic. The optimum temperature for vegetative growth of the fungus is about 24 °C. which closely approximates the optimum for disease incidence. Wilt symptoms appear slightly sooner and the temperature range of heavy disease incidence is broader (18° to 29 °C.) at high than at optimum soil moisture (21° to 27 °C.). The fungus persisted and was equally aggressive after three months in fine sandy loam, medium clay loam, and red clay loam under cropped and fallow conditions, except when the soil was fallow and dry during the period. Growing a susceptible host in infested soil for three months did not influence the activity of Verticillium more than did an immune host. Inoculum in a resting condition must be incorporated in moist soil a few days before it can readily infect plants. The addition of green plant residues and two organic acids caused a slight suppression of fungous activity.Air and soil temperatures and precipitation for May, June, July, and August for 1939, 1940, 1941, and 1942 have been examined. It was found that soil temperatures high enough for disease development are not likely to be encountered before late June and then the disease developed seriously only when the soil moisture was uniformly high during the months of May, June, and July. The relative infrequency of serious outbreaks of Verticillium wilt on the Niagara Peninsula thus appears to be due to the low soil moisture that ordinarily pertains during that part of the growing season when soil temperatures are high enough to favour the fungus.
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