Panicum tennesseense Ashe is a perennial, coarse-textured, cool-season grass (similar in appearance to many warm-seasoned grasses), which grows in the southern coastal areas of New England and is commonly found in home lawns, roadsides, and meadows. In August 2002, a large stand of P. tennesseense with significant leaf spotting was identified in Washington County, RI. Lesions were excised, surface sterilized with 0.6% sodium hypochlorite, and plated on one-half-strength potato dextrose agar (PDA). Cultures were incubated at 25°C, and mycelia quickly covered plates within 3 days. After 2 weeks in darkness, plates were covered with white, aerial mycelium. When placed under near-UV light, no aerial mycelium was produced, and mycelia were a salmon color. After 5 weeks, separate, spherical, 0.5-mm-diameter beige sclerotia were produced submerged throughout the media and turned red within a few days. Sclerotia were not produced in the absence of near-UV light. Spores were not produced, and right-angled branching, characteristic of Rhizoctonia spp., was observed. Monilioid hyphae were also produced in culture. At the optimal growth temperature of 32°C, mycelia grew an average of 33.6 mm/day. Cells were stained using safranin, and the number of nuclei observed within young hyphal cells was five or seven. On the basis of the described mycelial and sclerotial characterisitics, the isolated fungus was identified as Rhizoctonia zeae (1,2). To demonstrate pathogenicity, plugs of isolated fungi were cut from PDA plates, placed on leaf blades at the junction with leaf sheaths, and incubated in a greenhouse humidity chamber at an average temperature of 32°C and 80% humidity. After 3 days, significant disease was observed and included blighting of newly emerged shoots, blighting and spotting on leaf sheaths, and severe spotting on leaf lamina. Whereas Rhizoctonia solani has been reported as a pathogen on P. tennesseense, to our knowledge, this is the first report of R. zeae induced leaf and sheath spot on this host (1).
References: (1) H. Couch. Diseases of Turfgrasses. Krieger Publishing Co., Malabar, FL, 1995. (2) B. Sneh et al. Identification of Rhizoctonia species. The American Phytopathological Society, St. Paul, MN. 1991.
