Dispersal of rice sheath blight fungus, Rhizoctonia solani AG-1 (IA), and subsequent disease development in paddy fields, from survey of vegetative compatibility groups

Genetic structure of populations of the rice-infecting pathogen Rhizoctonia solani AG-1 IA from China

ABSTRACT Sheath blight disease (SBD) on rice, caused by Rhizoctonia solani AG-1 IA, is one of the most devastating rice diseases on a global basis, including China (in Eastern Asia), the world's largest rice-growing country. We analyzed the population genetics of nine rice-infecting populations from China using nine microsatellite loci. One allopatric population from India (Southern Asia) was included in the analyses. In total, 300 different multilocus genotypes were found among 572 fungal isolates. Clonal fractions within rice fields were 16 to 95%, suggesting that sclerotia were a major source of primary inoculum in some fields. Global Phi(ST) statistics (Phi(ST) = 42.49; P </= 0.001) were consistent with a relatively high level of differentiation among populations overall; however, pairwise comparisons gave nonsignificant R(ST) values, consistent with contemporary gene flow among five of the populations. Four of these populations were located along the Yangtze River tributary network. Gene flow followed an isolation-by-distance model consistent with restricted long-distance migration. Historical migration rates were reconstructed and yielded values that explained the current levels of population subdivision. Except for one population which appeared to be strictly clonal, all populations showed evidence of a mixed reproductive mode, including both asexual and sexual reproduction. One population had a strictly recombining structure (all loci were in Hardy-Weinberg equilibrium) but the remaining populations from China and the one from India exhibited varying degrees of sexual reproduction. Six populations showed significant F(IS) values consistent with inbreeding.

Survival and Subsequent Dispersal of Rice Sclerotial Disease Fungi, Rhizoctonia oryzae and Rhizoctonia oryzae-sativae, in Paddy Fields

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.