First report of Rhizoctonia zeae on turfgrass in Ontario

Abstract

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 leaves with symptoms were surface sterilized in 1% hypochlorite, and transferred to potato dextrose agar (PDA) amended with streptomycin. After one week at 25°C, the plates contained white colonies 5 cm diameter. 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, together with spherical orange sclerotia on sheaths. Affected leaves were incubated on PDA, and hyphae of R. zeae were recovered. In Canada, R. zeae has been reported from turfgrass samples in British Colombia (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 variously hot weather brown patch, leaf and sheath spot/rot/blight or brown ring patch. 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. We are grateful for the financial support of the Natural Sciences and Engineering Research Council of Canada, the Ontario Ministry of Agriculture and Food, as well as technical support from Darcy Olds and Russ Gowan.