Abstract
Rhizoctonia solani, Fusarium oxysporum and Pythium aphanidermatum in the saprophytic behaviour usually inhabited on debris in 0-20cm of soil depth, and P. aphanidermatum was in upper layer, R. solani in middle and F. oxysporum in deeper of soil, respectively. But, in overwetting soil, many of them immigrated into upper layer than in moderately moist soil. In each depth of soil phosphate and nitrogen decreased gradually with depth but carbon hydrate decreased suddenly at 30cm of depth. The survival ratio of R. solani inoculated in soil dug up from different depth decreased suddenly downwards of 30cm at 3 weeks after inoculation. It is considered that, carbon sources in soil determine the inhabitation of the fungi. P. aphanidermatum, inoculated in soil with rice straw, decreased in a half for 1 month, R. solani did for about 3.5 monthes and F. oxysporum did for about 5 monthes. Whereas P. aphanidermatum was lost after 3 monthes on straw in summer, a few of R. solani and F. oxysporum were surviving on it for 7 monthes. These 2 pathogens were dead more rapidly in submerged soil than in moderately moistened soil. P. aphanidermatum grew well on fresh straw in soil, but R. solani and F. oxysporum grew well on decomposed one by soil microben.
From these results it is suggested that, though these 3 pathogens are alive in soil by utilization of plant debris, one of the factors determinating the colonized and active survival abilities of them are the components of debris on which they arrive and grow.
