First Report of Rhizoctonia zeae on turfgrass in Ontario

In May 2004, a disease appeared on Poa annua and Agrostis stolonifera at a golf course near Toronto. Narrow yellow rings enclosing areas up to 30 cm across appeared after air temperatures reached 25°C. The disease resembled yellow patch caused by Rhizoctonia cerealis, but the weather was too warm for normal occurrences of that disease. The rings persisted until the end of July. In late May 2005, the disease appeared again after the weather became hot. A mixture of azoxystrobin and chlorothalonil was applied which seemed to suppress the disease within a week, until it reappeared in July. Samples were collected, and symptomatic leaves were surface sterilized in 1% hypochlorite, and plated onto potato dextrose agar (PDA) amended with streptomycin. After one week at 25C, the plates contained white colonies 5 cm across. Sequencing of the ITS region of ribosomal DNA showed a 99.6% match with a R. zeae sequence in GenBank. Pathogenicity was tested by inoculating 2-wk-old plants with mycelial plugs of R. zeae. Within one week at 25°C, significant blighting on leaves and sheaths was observed, as well as spherical orange sclerotia on sheaths. Symptomatic leaves were plated on PDA, and hyphae of R. zeae were recovered. In Canada, R. zeae has been reported from turfgrass samples in B.C. (Joshi, 2004), but this is the first report from Ontario. In the USA, this fungus has been reported to cause diseases of several turfgrass species (Smiley et al., 2005), which have been called hot weather brown patch, leaf and sheath spot/rot/blight or brown ring patch among others. We propose that it should be referred to as sheath spot. The taxonomy of this organism is similarly confused since R. zeae is considered to be a subspecies of Waitea circinata which contains at least two other subspecies including R. oryzae (Oniki et al., 1985; Leiner & Carling, 1994). More work is required to clarify the taxonomic disposition of R. zeae.