Characterization of Rhizoctonia solani Isolates Associated with Patch Diseases on Turfgrass

Cultural characteristics and pathogenicity of Rhizoctonia solani isolates obtained from brown patch on creeping bentgrass, Agrostis pulustris Huds. and large patch on zoysiagrass, Zoysia japonica Steud, were evaluated and compared with known R. solani anastomosis groups: AG-2-2III-B, AG-2-2IV, AG-1-IA, AG-4, and AG-5. Bentgrass and zoysiagrass isolates were obtained from infected grass leaf sheaths along disease patch margins. The bentgrass and zoysiagrass isolates differed culturally from one another. The bentgrass isolate and the AG-2-2IIIB tester both showed irregular clusters of myce-lia (not sclerotia), concentric zonation, dark brown main hyphae, and sparse aerial hy-phae on potato dextrose agar after two weeks of incubation at 22°C, 12h/12h light/dark. These two isolates caused high levels of disease on creeping bentgrass cv. Crenshaw in in vitro pathogenicity tests. The zoysiagrass isolate most closely matched R. solani AG-2-2IV in both cultural characteristics and pathogenicity on creeping bentgrass cv. Crenshaw. The zoysiagrass isolate and the AG-2-2IV tester both had abundant aerial hyphal growth, dark brown main hyphae, and no sclerotial formation or zonation on potato dextrose agar after two weeks of incubation. Optimum temperature for growth of both isolates was 25°C but unlike the bentgrass isolate and the AG-2-2IIIB tester, the zoysiagrass isolate and the AG-2-2IV tester did not grow at 35°C. The zoysiagrass isolate and the AG-2-2 IV tester caused low levels of disease on creeping bentgrass cv. Crenshaw in in vitro pathogenicity tests. Results indicate that cultural characteristics and host range of the bentgrass isolate and those of the zoysiagrass isolate are different. Isolates representing R. solani AG-2-2IIIB and AG-2-2IV were tested for sensitivity to azoxystrobin in in vitro tests. Sensitivity to azoxystrobin (effective concentration causing 50 % growth inhibition [EC50]) was determined by radial growth on potato dextrose agar amende