SEASONAL DEVELOPMENT OF DOLLAR SPOT EPIDEMICS IN MARYLAND AND NITROGEN EFFECTS ON FUNGICIDE PERFORMANCE IN CREEPING BENTGRASS
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Dollar spot (Sclerotinia homoeocarpa) is a common and destructive disease of creeping bentgrass (Agrostis stolonifera). The frequency and severity of dollar spot epidemics has not been quantified and there are no effective predictive models. High rates of nitrogen (N) reduce dollar spot injury, but low N rates applied in summer have not been assessed for disease suppression. Field studies were conducted from 2008 to 2010 with the following objectives: a) to describe the relationship among season, environmental factors and the severity of dollar spot epidemics in six creeping bentgrass cultivars; b) to evaluate six water soluble N sources applied at a low rate (7.3 kg N ha-1) in summer for their impact on dollar spot severity; and c) to assess the performance of low fungicide rates tank-mixed with N on dollar spot severity. Two epidemics were observed each year between spring and mid- autumn, with the second being most severe. A third, late autumn epidemic also was observed in each year. The first epidemic in May was effectively predicated using a degree day model having a biofix date of 1 April and a 15°C base temperature. Ammonium sulfate was most consistently effective in reducing dollar spot injury, but caused foliar injury. Tank-mixing a low chlorothalonil rate with N generally reduced fungicide efficacy.