INTERACTIVE EFFECTS OF PLANT-GROWTH REGULATORS AND FUNGICIDES ON EPIDEMICS OF DOLLAR SPOT IN CREEPING BENTGRASS

Studies were conducted to assess interactions among gibberellin inhibi tor plant growth regulators (PGRs) and fungicides on dollar spot of cr eeping bentgrass caused by Sclerotinia homoeocarpa. The PGRs paclobutr azol and flurprimidol were fungistatic to S. homoeocarpa in vitro at s ignificantly lower concentrations (EC(50) < 0.5 mu g/ml) than was the PGR trinexapac-ethyl (EC(50) > 15 mu g/ml). In the field, paclobutrazo l, flurprimidol, and trinexapac-ethyl were applied to creeping bentgra ss at the respective rates of 0.16, 0.33, and 0.19 kg a.i./ha either a lone or as pretreatments 4 days prior to applications of the fungicide s 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 = amou nt of disease and t = time) were significantly lower in bentgrass trea ted with flurprimidol in 1993 and 1994 or with paclobutrazol in 1994 c ompared to nontreated turf and turf treated with trinexapac-ethyl. Sim ilar effects were detected from analysis of areas under disease progre ss curves (AUDPC). The trinexapac-ethyl treatment had no significant e ffect on epidemic rates or values of AUDPC compared to nontreated cont rols in 1993 and 1994. Based on values of AUDPC and/or time to reach 5 % disease severity, pretreatment of turf with flurprimidol significant ly enhanced the efficacy of chlorothalonil, iprodione, and propiconazo le in 1994 and each of the four fungicides tested in 1995. Pretreatmen t with paclobutrazol resulted in effects similar to those observed wit h the flurprimidol pretreatment in 1994, but paclobutrazol did not enh ance the efficacy of chlorothalonil or thiophanate-methyl in 1995. Pre treatment with trinexapac-ethyl significantly enhanced the efficacy of chlorothalonil, iprodione, and propiconazole in 1994, but had no effe ct on fungicide efficacy in 1995. Foliar clipping weights were signifi cantly greater in plots treated with any of the PGRs 28 days after tre atment in 1993 compared to nontreated controls. In 1994, canopy height s of bentgrass were significantly lower in PGR-treated plots 10 and 18 days after treatment compared to nontreated controls.