Abstract
In 1998 to 2001, a commercial rice (Oryza sativa) paddy field (area: ca. 0.14 ha) was surveyed for Rhizoctonia oryzae and R. oryzae-sativae, causal agents of bordered sheath spot and brown sclerotium disease of rice plants, respectively, to determine their survival in soil and stubble during the pretransplanting season, and the effect on disease development during the maturation season of rice. Then, infection by these fungi of weeds belonging to 17 families, which grew in the four neighboring fields, was examined during rice growing seasons. All field isolates of the fungi from soil, stubble, rice sclerotial disease lesions (diseased tissues), and weeds were assorted into mycelial compatibility groups (MCGs) based on the barrage zone reaction of paired isolates. In R. oryzae, 3 to 8 MCGs were annually found from soil/stubble, 2 to 4 MCGs from rice bordered sheath spot lesions, and 4 to 9 MCGs from 4 to 14 weeds. MCGs common to both soil/stubble and diseased tissues, soil/stubble and weeds, and diseased tissues and weeds numbered 1 to 2 in all cases. In R. oryzae-sativae, MCGs common to both soil/stubble and brown sclerotium disease lesions, and soil/stubble and weeds, numbered 1 to 4 and 0 to 5, respectively. In R. oryzae and R. oryzae-sativae, a few MCGs were common to soil/stubble, diseased tissues and weeds, and some were also common to diseased tissues in 1998 and soil/stubble in 1999, or weed in 1998 and diseased tissues in 1999. Members belonging to a single MCG from rice diseased tissues were detected from maximally five weeds growing in the neighboring fields. These results indicate that fungi that had caused sclerotial diseases at the maturation stage of rice plants survived on and in soil and stubble until the pretransplanting season of the next year, followed by wide dispersal in and out of fields and by infection and disease development on rice plants and various weeds.
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