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First Report of Dollar Spot, Caused by Sclerotinia homoeocarpa, on Poa pratensis in Saskatchewan, Canada

Dollar spot disease affects many species of grasses in North America but has not been previously reported from Saskatchewan. In July 2000, symptoms were observed on golf course fairways in Saskatoon. No dollar spot disease was observed on adjacent putting greens or tees composed of Agrostis palustris (creeping bentgrass), perhaps because the tees and greens were grown under a higher nitrogen fertility regime. Fungicide treatments are usually not required for turf disease control during the warm, dry summer growing season in Saskatoon, and no fungicides had been applied at this location. Daytime temperatures near 25°C and heavy dew at night preceded the disease outbreak, which affected about 5% of the turf across large areas of several fairways. The fairways were originally seeded to Festuca rubra subsp. rubra (common creeping red fescue) and Poa pratensis (Kentucky bluegrass) but also contained annual bluegrass (P. annua). The disease was observed on leaf blades of all three species. In addition to 5-cm-diameter circular patches of brown grass, sharply delimited individual leaf lesions and cobweb-like aerial mycelia on the grass were observed. Fungal isolates were obtained by plating infected P. pratensis leaf blades on potato dextrose agar and then transferring to oat agar. On oat agar, isolates produced white, fluffy aerial mycelium, columnar when mature, and usually with a cinnamon base and dark brown stromata or sclerotial flakes on and in the agar. DNA was extracted from an isolate and amplified using the primers ITS1 and ITS4 (1). A 500-bp fragment presumed to contain the internal transcribed spacer region of the ribosomal DNA (ITS) was purified and sequenced (1), and it showed 96% identity with the ITS of a Sclerotinia homoeocarpa isolate, Genbank accession AF067640. To test Koch's postulates, P. pratensis cv. Loft 1757 was grown from seed in 15 ml of sand at 20°C under constant fluorescent light. Two-week-old turf was inoculated with 5-mm-diameter mycelial plugs of the fungus from 1-week-old cultures on potato dextrose agar by placing inoculum plugs on the sand. Inoculated turf was incubated in a loosely-lidded clear plastic container at 20°C under constant fluorescent lighting. After 1 week, lesions and white aerial hyphae were observed on the leaf blades, while no disease was observed on the uninoculated controls. The fungus was reisolated from foliar lesions to complete Koch's postulates. In addition to P. pratensis, both P. annua and A. palustris cv. Penncross were also inoculated and showed disease symptoms, with greater disease severity on P. annua.

Reference: (1) T. Hsiang and C. Wu. Mycol. Res. 104:16, 2000.