Bentgrass Susceptibility Affects Fungicide Programming for Dollar Spot.

Host susceptibility and fungicide application timing can influence control of dollar spot (caused by Sclerotinia homoeocarpa F.T. Bennett) of creeping bentgrass (Agrostis stolonifera L.) turf. This field study evaluated the effect of fungicide application timings on dollar spot incidence on susceptible (‘Independence’) and highly tolerant (‘Declaration’) cultivars. The trial used a 2 x 8 x 2 factorially arranged randomized complete block design with 5 blocks. Factors included bentgrass cultivar; initial fungicide timing [based on disease threshold, calendar, logistic regression model, or growing degree day (GDD) model (20-30, 30-40, 40-50, 50-60, or 60-70 GDD; base 15°C and biofix 1 April)]; and subsequent fungicide timing [logistic regression model or disease threshold]. A calendar program applying fungicide every 21-d from May to November (nine applications yr^-1) and an untreated control were included for each cultivar. All plots were inoculated once with S. homoeocarpa isolates NJDS003 and NJDS007 on 7 Apr. 2015 and fungicide treatments (boscalid at 0.38 kg a.i. ha^-1) were initiated May 2015 and 2016, and April 2017. Disease response to treatments was limited during 2016 due to unintended dollar spot suppression from fludioxonil. Cultivar was the most important factor influencing the area under disease progress curve during 2015 and 2017 (to-date). Cultivar interacted with subsequent fungicide timing to influence the level of disease control and total annual fungicide inputs during 2015. Logistic regression model- and disease threshold-based subsequent applications produced excellent disease control (< 3 infection centers m^-2) on ‘Declaration’; whereas, only the logistic-regression model produced consistently excellent disease control on ‘Independence’. Three threshold-based applications were made to ‘Declaration’ plots, regardless of the initial fungicide timing, during 2015; whereas, six or seven threshold-based applications were made to ‘Independence’, depending on the initial fungicide timing. Moreover, disease incidence was occasionally unacceptable (up to 8 centers m^-2) on ‘Independence’ threshold-based plots.