Effect of Water Temperature on Direct Germination of the Sporangia of Phytophthora infestans.

Effect of water temperature on conversion of the germinability from indirect to direct type of the sporangia of Phytophthora infestans was investigated. The sporangia produced on inoculated potato tuber slices were suspended in the tap water of Hokkaido Natl. Agric. Exp. Stn. which was very favorable for both indirect and direct germination. When the suspensions were incubated at the temperatures of 30-36°C, the ability of indirect germination was lost and the sporangia were converted to directly germinative type. The higher the water temperature was, the conversion occurred the more rapidly. This was also true between 20 and 30°C. By incubating the suspensions for 16-24hr at 33°C, almost all (>95%) sporangia could be converted to directly germinative type. By using such sporangia, effect of water temperature on the proportion and rapidity of direct germination was also investigated. The comparatively low temperatures of 10-17°C were optimum to achieve the highest proportion of direct germination (>95% in 48hr), while the comparatively high temperatures of 22-24°C were optimum to achieve the largest rapidity of direct germination, where nearly maximum proportions (about 60%) were attained in 24hr. Both the extreme temperatures of 3 and 28°C were inhibitory to direct germination.