Survival Durations of Sclerotium oryzae-sativae Sawada Strains in Paddy Fields and Their Distribution in a Neighbouring Field

Field isolates of Sclerotium oryzae-sativae Sawada were obtained from sheath blight-like lesions on maturing rice plants in 1983-1989 and were classified into strains according to anastomosis reactions (Hyphal fusion without cell death was observed in every pairing test between isolates within the same strain, but the fusion with cell death was observed in the test between isolates belong to different strains).Two hundred and eighteen and 245 isolates from two paddy fields differed in rice variety cultivated were assorted into 37 and 50 strains, respectively. Survey of presense of these strains in paddy fields for 6 years resulted that 55% of strains survived for one year and 22% of them were for 4-6 years. Strains distributing over a wide area in a field tended to survive for longer periods. In survival for 4-6 years, any strains did not remain at the same or neighbouring plot where those previously appeared. About 60% of strains were new ones which had not been detected in previous years, although the percentage differed in the year and the field. Three to 5 among 9-20 strains detected in each of two neighboring fields (ca. 20m apart) were common in the both fields in 1986 and 1987.