Interactive effects of plant growth regulators and fungicides on epidemics of dollar spot in creeping bentgrass

Studies were conducted to assess interactions among gibberellin inhibitor plant growth regulators (PGRs) and fungicides on dollar spot of creeping bentgrass caused by Sclerotinia homoeocarpa. The PGRs paclobutrazol and flurprimidol were fungistatic to S. homoeocarpa in vitro at significantly lower concentrations (EC(50) < 0.5 micrograms/ml) than was the PGR trinexapac-ethyl (EC(50) > 15 micrograms/ml). In the field, paclobutrazol, flurprimidol, and trinexapac-ethyl were applied to creeping bentgrass at the respective rates of 0.16, 0.33, and 0.19 kg a.i./ha either alone or as pretreatments 4 days prior to applications of the fungicides chlorothalonil, iprodione, propiconazole, and thiophanate-methyl at 9.6, 3.1, 0.4, and 3.1 kg a.i./ha, respectively. In plots treated with the PGRs alone, rates of dollar spot epidemics (dy/dt, where y = amount of disease and t = time) were significantly lower in bentgrass treated with flurprimidol in 1993 and 1994 or with paclobutrazol in 1994 compared to nontreated turf and turf treated with trinexapac-ethyl. Similar effects were detected from analysis of areas under disease progress curves (AUDPC). The trinexapac-ethyl treatment had no significant effect on epidemic rates or values of AUDPC compared to nontreated controls in 1993 and 1994. Based on values of AUDPC and/or time to reach 5% disease severity, pretreatment of turf with flurprimidol significantly enhanced the efficacy of chlorothalonil, iprodione, and propiconazole in 1994 and each of the four fungicides tested in 1995. Pretreatment with paclobutrazol resulted in effects similar to those observed with the flurprimidol pretreatment in 1994, but paclobutrazol did not enhance the efficacy of chlorothalonil or thiophanate-methyl in 1995. Pretreatment with trinexapac-ethyl significantly enhanced the efficacy of chlorothalonil, iprodione, and propiconazole in 1994, but had no effect on fungicide efficacy in 1995. Foliar clipping weights were significantly greater in plots treated with any of the PGRs 28 days after treatment in 1993 compared to nontreated controls. In 1994, canopy heights of bentgrass were significantly lower in PGR-treated plots 10 and 18 days after treatment compared to nontreated controls.