Characterization, Pathogenicity and in Vitro Sensitivity of Rhizoctonia Spp. Associated with Leaf and Sheath Spot of Bermudagrass Putting Greens in NC

Twenty isolates of a fungus resembling R. zeae were obtained from three golf courses with bermudagrass greens that were exhibiting leaf and sheath spot symptoms in 2013 and 2014. Morphological and molecular identification showed that 17 isolates were identified as R. zeae and 3 were identified as W. circinata var. zeae. Pathogenicity of the 20 isolates was determined by inoculating hybrid bermudagrass (cv ‘Champion’) plants in a growth chamber. After inoculation, plants were incubated in a growth chamber at 34/30 °C (day/night) and sealed in a closed container to ensure 100% humidity. After seven days, disease was visually estimated using a modified Horsfal-Barrett scale (0-10). All isolates were pathogenic to the hybrid bermudagrass cultivar ‘Champion’ and aggressiveness among isolates varied. Sensitivity of 7 isolates collected in 2013 to 16 fungicides (flutolanil, pyraclostrobin, fluoxastrobin, azoxystrobin, propiconazole, triticonazole, triadimefon, penthiopyrad, fluxapyroxad, azoxystrobin+difenoconazole, tebuconazole, difenoconazole, iprodione, polyoxin-D zinc salt, fluxapyroxad+pyraclostrobin, and chlorothalonil) was tested. The sensitivity of 13 isolates collected in 2014 was tested to 5 fungicides (chlorothalonil, azoxystrobin, pyraclostrobin, difenoconazole, and flutolanil). In general, all the isolates were extremely sensitive to DMI and SDHI fungicides (EC₅₀ < 1 mg a.i. L^-1). All isolates showed insensitivity to QoI fungicides (EC₅₀ > 10 mg a.i. L^-1). These results demonstrated that R. zeae is a pathogen of ultradwarf bermudagrass under the imposed environmental conditions. R. zeae is sensitive to many fungicides, but based on in vitro sensitivity data presented here some may not perform well under field conditions.