Wednesday, 9 November 2005
8

Distribution and Management of Fungicide Resistance in North Carolina Populations of Sclerotinia homoeocarpa.

Lane Tredway, North Carolina State University, 2518 Gardner Hall, Raleigh, NC 27695

Dollar spot, a foliar disease caused by Sclerotinia homoeocarpa, continues to be one of the most severe diseases of golf course turfgrasses. A survey was conducted to determine the distribution of fungicide resistance in populations of this pathogen from putting greens and fairways in North Carolina. Isolates (30-50 per location) were collected from 15 locations, including 13 exposed populations from golf courses and 2 unexposed baseline populations. Baseline populations contained low frequencies of thiophanate-methyl resistance (5% and 10%), but 95% of isolates from exposed populations were resistant to this fungicide. Resistance to iprodione was detected in one location on Poa annua putting greens in Western NC. Mean EC50 values for propiconazole were 0.017 and 0.019 in baseline populations. Mean EC50 values for exposed populations ranged from 0.018 (resistance factor = 1) to 0.068 (resistance factor = 3.78). Four separate field experiments were inoculated on 18 Apr 2005 with a mixture of S. homoeocarpa isolates with mean propiconazole EC50 values of 0.011, 0.020, 0.033, or 0.054. Experiments were mowed separately to prevent spread among plots inoculated with different isolates, and fungicide-treated buffer areas were maintained between experiments. Fungicide treatments were applied on a curative basis on 3 May 2005 and 26 May 2005. All fungicides provided excellent curative control of the S. homoeocarpa population with mean EC50 value of 0.011. Reduced propiconazole efficacy was observed in populations with mean EC50 values of 0.020, 0.033, and 0.054. Applications of boscalid and iprodione were generally less effective for control of populations with mean EC50 values of 0.033 and 0.054 ppm. Reduced efficacy of curative DMI applications may occur at EC50 values as low as 0.020, and the curative control provided by other fungicide classes may also be impacted by shifts in DMI sensitivity.

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