Laboratory of Plant Pathology, Faculty of Agriculture, Gifu University, Gifu 501-11, Japan
Prevalence and sites of survival of Rhizoctonia solani AG2-2 LP were studied in zoysia grass for 6 years. AG2-2 LP isolates commonly were recovered over all seasons at sites with a history of large patch disease. In patch margins, AG2-2 LP isolates were recovered from crowns of zoysia grass regardless of whether the disease occurred, but were most frequently isolated from the sheath tissues during disease occurrence. In healthy sites approximately 30 cm from the patch, isolates were obtained before but not during disease occurrence. Once disease occurred, patch symptoms rapidly expanded to the edge of tissue colonized by the pathogen during autumn to early spring. To verify that the pathogen spread to healthy areas, the clonal relationship among isolates was examined using their anastomosis reaction. Isolates recovered from the patch and healthy area outside the patch were of the same clone, but isolates from different patches differed. Cultural characteristics and pathogenicity of the AG2-2 LP isolates were compared with R. solani AG2-2 IIIB and R. solani AG2-2 IV. The AG2-2 LP isolates showed an irregular cluster of mycelia (not sclerotia), an irregular zonation, dark brown main hyphae, and sparse aerial hyphae on potato dextrose agar after 4 weeks of incubation. Optimum temperature for growth was 23°C. Cultural characteristics of AG2-2 subgroups IIIB and IV differed from LP isolates. All isolates of AG2-2 LP caused moderate to high levels of disease on zoysia grass, but were nonpathogenic or caused little disease on bent grass and sugar beet. These results indicate that cultural characteristics and host range of AG2-2 LP are different than those of AG2-2 IIIB and AG2-2 IV.