ABSTRACT
Isolates of an unidentified Rhizoctonia sp. (UN isolates) were obtained from Japanese zoysia grass (Zoysia japonica Steud) that exhibited symptoms of a new sheath rot disease. UN isolates were binucleate and showed hyphal fusion with tester isolates of Rhizoctonia anastomosis group (AG)-D. Those isolates were compared with isolates of subgroups I and II of Rhizoctonia AG-D based on cultural morphology, hyphal growth rate at different temperatures, anastomosis frequency, pathogenicity, and sequence analysis of the internal transcribed spacer (ITS) region of ribosomal DNA genes (rDNA-ITS region). The mycelial color of UN isolates was light yellow which differs from AG-D I but is similar to AG-D II. Sclerotia of UN isolates were dark brown in color and larger in size (1 to 3 mm in diameter) than those of AG-D subgroup I (1 mm in diameter), whereas isolates of AG-D II produced white mycelial clamps 4 to 5 mm in size. Hyphal growth rate of UN isolates was slower than that of two AG-D subgroups at several temperatures, especially 25°C. In pathogenicity tests on Japanese zoysia grass, UN isolates showed moderate disease severity and lower pathogenicity than isolates of AG-D subgroups I and II. Sequences of the rDNA-ITS region within UN isolates were almost homologous, but had lower homology with subgroups AG-D I or II. Phylogenetic trees constructed using ITS sequences showed that UN isolates formed an individual cluster that differed from the clusters of the two subgroups. We propose that UN isolates are a new subgroup of Rhizoctonia AG-D, subgroup III, and the name of the disease is "spring-rot" on Japanese zoysia grass.
ABSTRACT
Isolates of an unidentified Rhizoctonia sp. (NP isolates), obtained from creeping bentgrass (Agrostis stolonifera var. palustris) in Japan that exhibited symptoms of a new disease, were compared with isolates of three varieties of Waitea circinata var. oryzae, var. zeae, and var. circinata. NP isolates also were compared with isolates of R. oryzae obtained from creeping bent-grass exhibiting white patch-like symptoms (RW isolates). The color and size of sclerotia, color of mycelia, and pigment deposition of NP isolates was similar to that of RW isolates and W. circinata var. circinata, but distinctly different from W. circinata var. oryzae and W. circinata var. zeae. The optimal temperature for hyphal growth of NP isolates, RW isolates, and W. circinata var. circinata was 28°C, and for W. circinata var. oryzae and W. circinata var. zeae was 30°C. Pathogenicity tests on creeping bentgrass showed that the severity of disease caused by NP isolates, RW isolates, and W. circinata var. circinata was greater than with W. circinata var. oryzae, but lower than with W. circinata var. zeae. No significant differences in symptom expression were apparent among NP isolates, RW isolates, and W. circinata var. circinata. A phylogenic tree, obtained using the results of random amplified polymorphic DNA-polymerase chain reaction (RAPD-PCR), showed that isolates of W. circinata var. oryzae and W. circinata var. zeae separated into individual clusters, while NP isolates, RW isolates, and W. circinata var. circinata clustered together. The lengths of the rDNA internal transcribed spacer (ITS) region of NP isolates, RW isolates, and W. circinata var. circinata were identical but smaller than those of W. circinata var. oryzae and W. circinata var. zeae. Restriction fragment length polymorphism (RFLP) analysis of the rDNA-ITS region, using three enzymes (HapII, HinfI, and MboI), also showed that NP isolates were the same as RW isolates and W. circinata var. circinata, but different from W. circinata var. oryzae and W. circinata var. zeae. Based on these results, the NP isolates causing a new disease on bentgrass are W. circinata var. circinata, and that RW isolates are also W. circinata var. circinata but not R. oryzae. We propose that the name of the disease on creeping bentgrass caused by W. circinata var. circinata is brown ring patch.