In Vitro Sensitivity of Rhizoctonia Zeae, the Fungus Associated with Mini-Ring Disease of Ultradwarf Bermudagrasses, to 16 Different Fungicides.

An unusual disease develops on ultradwarf bermudagrasses during late July through early October in NC. The disease is typified by small tan spots ranging from 2.5 cm to 10 cm in diameter that expand into necrotic or tan rings ranging from 10 cm to 45 cm in diameter.  Once symptoms develop, they persist throughout the winter months and usually do not recover until spring-green up the following year.  Fungicides have been ineffective in numerous field trials in NC and SC most likely because little information exists about the etiology and epidemiology of the disease.  Seven isolates resembling Rhizoctonia zeae were collected and their sensitivity to 16 different fungicides (flutolanil, pyraclostrobin, fluoxastrobin, azoxystrobin, propiconazole, triticonazole, triadimefon, penthiopyrad, fluxapyroxad, azoxystrobin+difenoconazole, tebuconazole, iprodione, polyoxin-D zinc salt, fluxapyroxad+pyraclostrobin, chlorothalonil) were measured in vitro. In the laboratory, each fungicide was added to PDA that was cooled to 50^oC at the following concentrations 0, 0.001, 0.01, 0.1, 1, and 10 ppm (mg AI kg^-1). Five mm plugs of agar and mycelium from actively growing cultures of R. zeae were placed on the media in the center of dishes.  Radial growth was measured 24h, 48h and 72h after placement in a dark incubator.  Each concentration was replicated three times and the entire experiment was repeated three times. All of the isolates collected were most sensitive to the DMI fungicides (difenoconazole, tebuconazole, propiconazole, triticonazole, triadimefon), dicarboximide (iprodione) and combined fungicides (azoxystrobin+difenoconazole, fluxapyroxad+pyraclostrobin). SDHI fungicides (flutolanil, penthiopyrad, fluxapyroxad) and aniline fungicide (chlorothalonil) were not as effective suppressing mycelial growth.  The QoI fungicides (pyraclostrobin, fluoxastrobin, azoxystrobin) and polyoxin-D did not suppress mycelial growth when compared to the non-amended controls.  This work demonstrates that the fungus associated with leaf and sheath blight/mini-ring is sensitive to numerous fungicides.  This work demonstrates that timing of fungicide applications in field trials is extremely important.