Outbreaks of Rice Sclerotial Diseases Caused by Rhizoctonia and Sclerotium Species in Paddy Fields of Early Season Culture in Aichi Prefecture

In a western rice growing area of Aichi Prefecture in central Japan, five rice scletotial diseases, i. e., sheath blight caused by Rhizoctonia solani Kuhn (AG 1, IA) and 4 sheath blight-like diseases caused by R. solani (AG 2-2, III B), R. oryzae Ryker et Gooch, Sclerotium fumigatum Nakata and S. oryzae-sativae Sawada, were comparatively surveyed for 3 years from 1989 to 1991 concerning outbreak of the diseases in paddy fields of early (ESC) and normal (NSC) season culture. In NSC fields, numerous outbreaks of sheath blight and 2 diseases caused by S. oryzae-sativae and S. fumigatum were detected at maturation (full ripe-stage) of rice plants. In ESC fields, on the other hand, sheath blight and bordered sheath spot caused by R. oryzae were frequently detected at maturation (milk ripe-to yellow ripe-stage), but 2 diseases caused by S. oryzae-sativae and S. fumigatum, especially the latter, were less frequently observed. In the course of rice growth phases from the internode elongation- (July 3) to the yellow ripe-stage (Aug. 7), intermittent surveys concerning disease development at intervals of 11-12 days in 3 ESC fields indicated that initial outbreaks of bordered sheath spot and the disease caused by S. oryzae-sativae occur at the phase from the internode elongation to the heading-stage and from the heading-to the milk ripe-stage, respectively.