Kimiharu Inagaki

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

The rice sheath blight fungus,Rhizoctonia solani AG-1(IA), was isolated from surface soil and stubble in the pretrans-planting season (early March) and from sheath blight lesions on plants at the maturing stage (late September) in two paddy fields (TW and TM:8 a). Isolates were classified into vegetative compatibility groups (VCG) based on the formation of a barrage zone (or interaction zone) at the colony interface between isolates on paired cultures. In field TW surveyed in 1991 and 1992, 128–142 isolates obtained in the pretransplanting season and 82–85 isolates from lesions on mature plants were classified into 27–29 and 28–30 VCGs, respectively. Seven to 10 VCGs (26–35%) were common between both samples, and of 30 VCGs from mature plants 12 (40%) survived the winter 1991/1992 and recovered from stubble and/or soil. In field TM in 1992, 111 isolates in the pretransplanting season and 99 from mature plants were divided into 18 and 14 VCGs, respectively. Seven (39%) out of 18 VCGs in the pretransplanting season were recovered again from mature plants. Occurence of common VCGs throughout the year indicated that ca. one third of VCGs found in the pretransplanting season caused sheath blight on rice plants about 6 mo later in the same field. Further, it was shown that about the same rate of VCGs which had caused disease overwintered on stubble and/or surface soil until the following pretransplanting season.

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.

Molecular markers for genotyping anastomosis groups and understanding the population biology of Rhizoctonia species

Soil-borne Rhizoctonia fungi cause serious diseases in several plant species. For the classification of these fungi, the number of nuclei in a hyphal cell and the anastomosis reaction are important criteria. Although Rhizoctonia spp. has a wide host range, the causal agents have been reported to be selective for host plant families or species and lead to severe disease. Reports of new diseases, particularly in new host plants, and severe damage in agricultural fields incurred by subdivided or newly found groups of Ceratobasidium and Waitea circinata (a varied teleomorph of Rhizoctonia) have been increasing in recent years. The food production environment is altering because of climate change, introduction of potential new host plants, and heavy use of chemicals that reduce microbial diversity. These changes favor the occurrence of new diseases incurred by undefined anastomosis groups (AGs) or subgroups of Rhizoctonia spp. On the basis of the phylogenetic relationships of AGs and subgroups in Rhizoctonia spp., molecular markers for discriminating the groups of the Rhizoctonia species complex have been developed. The application of genetic markers, in the form of microsatellites or simple sequence repeats (SSR), has become increasingly important in fungal genetics. The analyses of population genetics for Rhizoctonia spp. using SSR markers elucidated the modes of sexual and asexual reproduction, phylogeographical distributions, and global migrations associated with adaptation to agroecosystems.

Annual Changes in Outbreak of Rice Sclerotial Diseases Caused by Rhizoctonia and Sclerotium spp. in Paddy Fields

From 1981 to 1989, outbreaks of Rhizoctonia fumigata-, R. oryzae-sativae-, R. oryzae-, R. solani (AG 2-2, IIIB)-and Sclerotium. hydrophilum-disease were surveyed during the maturation stage of rice plants in 2 paddy fields. The surveys were carried out once a year by collecting sheath blight-like lesions from 30-35 plots (4.5-5.0 m interval between plots) in the fields (5-6 a) and by isolating causal fungi from these lesions. R. oryzae-sativae-caused brown sclerotial disease differed in the outbreak area over time, although the other 4 sclerotial diseases did not vary markedly in the outbreak area. R. fumigata -caused disease , especially, and brown sclerotial disease occurred frequently over a wide area, while R. oryzae-caused sheath spot, R. solaniand S. hydrophilum-caused disease were limited in area. For sheath spot, there was continuous occurrence in the same plot during the surveys. Brown sclerotial disease was also detected in a few of the same divisions or neighboring ones where the disease had occurred in preceding years.