Isolation of Pythium Species from Zoysia Grass and Their Effect on Severity of Large Patch Disease

Pythium periplocum, P. rostratum, P. torulosum, and P. vanterpoolii were predominant Pythium species isolated from nine sites with a history of large patch disease of zoysia grass. Rhizoctonia solani AG2-2 LP and the Pythium species were isolated from 21 sod samples of zoysia grass exhibiting large patch symptoms in five golf courses. R. solani AG2-2 LP was obtained from all samples, while P. periplocum, P. rostratum, P. torulosum, and P. vanterpoolii were obtained from 14, 6, 11, and 8 samples, respectively. At least one of the four Pythium species was recovered from 19 samples. To verify pathogenicity of these four species of Pythium on zoysia grass, they were inoculated alone and together with R. solani AG2-2 LP on zoysia grass. When individual isolates were used to inoculate zoysia grass, R. solani AG2-2 LP, P. periplocum, and P. vanterpoolii were moderately aggressive, while P. torulosum and P. rostratum caused little or no disease. Symptoms produced by R. solani AG2-2 LP included orange discoloration of the sheath, and the sheath was easily pulled from the crown. P. periplocum and P. vanterpoolii induced only sheath chlorosis, and the sheath was not easily removed from the crown. In coinoculation tests, the combination of R. solani AG2-2 LP and P. torulosum intensified disease severity on zoysia grass and induced more rapid symptom development than did R. solani AG2-2 LP alone. The combination of R. solani AG2-2 LP and P. periplocum or P. vanterpoolii resulted in sheath necrosis and bare patches, similar to large patch symptoms observed on golf courses.