Relatedness among Isolates of Rhizoctonia oryzae Ryker et Gooch Causing Bordered Sheath Spot and Annual Vicissitude of the Fungus in Paddy Fields

Isolates of bordered sheath spot fungus, Rhizoctonia oryzae Ryker et Gooch, obtained from sheath lesions within a single rice hill had the same cultural characteristics on potato sucrose agar (PSA), and these isolates belonged to the same anastomosis group without death of fused cells. Therefore, this disease within a single rice hill seemed to be caused by the same fusion group. On the other hand, populations of fusion groups in the field (area: 6-9a) were 1 to 6. Some fusion groups were found in the same field during 3-6 years. The fusion groups distributed widely in the fields were expected to survive and continue to cause disease in years to come. Epidemiological examination of fusion groups in paddy fields are needed to understand the development of sclerotial diseases.