Ecologic and Taxonomic Studies on Pythium as Pathogenic Soil Fungi : VIII. Differences in Pathogenicity of Several Species of Pythium

1. All of the Pythium used, except P. monospermum and P. vexans, was highly pathogenic on tomato, watermelon, Hibiscus manihot and morning-glory seedlings. Although P. monospermum and P. vexans failed to infect or were considerably weakly pathogenic on the seedlings of squash (Amakuri), kidney bean and rice plant, they were highly pathogenic on sweetpotato root and potato tuber tissues. 2. The root tissue of sweetpotato (Norin No. 1) was highly susceptible to different species of Pythium. 3. Sweetpotato root (Gokoku) was resistant to P. ultimum, whereas the roots (Norin Nos. 1, 4, 6, 10, 16; Kokei No. 14; Kanto No. 23) were highly susceptible. Such roots as Norin No. 3, Kokei No. 3, Kanto (Nos. 31, 32, 33, 34, 35) and Hichi-fuku became resistant to the fungus near the optimum temperature for their growth. Minimum and maximum temperatures for disease development were below 10℃ and above 36℃, respectively. 4. Pythium with the exception of P. spinosum and P. ultimum (Stock culture no. 11) was generally pathogenic on potato tuber. All the potato tuber varieties used were susceptible to P. ultimum