Dollar spot, caused by Sclerotinia homoeocarpa F.T. Bennett, is considered the most economically important disease of golf course turf in the northern United States (Vargas, 2005). Leaf wetness in the form of dew (condensate and plant-generated moisture, including wound exudates and guttation fluid) plays an important role in the development of dollar spot, and displacement of dew in early morning can reduce symptoms by interrupting prolonged leaf wetness required for disease development (Delvalle et al., 2011; Ellram et al., 2007; Nikolai et al., 2001; Williams et al., 1996). In addition to the use of dew removal and other cultural practices, fungicides often are applied throughout the growing season to provide adequate dollar spot disease control.
Mowing practices and dew removal strategies associated with fungicide performance for dollar spot control in creeping bentgrass (Agrostis stolonifera L.) have been evaluated recently. Putman and Kaminski (2011) found that mowing frequency (e.g., 2, 4, or 6 d/week) had no influence on fungicide performance. In another study, Delvalle et al. (2011) reported that dollar spot control with fungicides can be extended by daily dew removal or increased mowing frequency. Increased mowing frequency likely results in a reduction of dollar spot as a result of dew removal as well as physical disruption and removal of inoculum (Delvalle et al., 2011; Pigati et al., 2010).
Golf course superintendents often apply pesticides early in the morning as a result of concerns of potential pesticide exposure to golfers, drift to non-target areas, and to avoid interfering with play. Significant amounts of dew may be present on turf surfaces in early morning and may influence the performance of certain pesticides if not displaced before application. Although research shows benefits associated with routine dew removal, information on dollar spot control related to the presence of dew at the time of fungicide application is limited. McDonald et al. (2006) reported that in most cases no significant differences in dollar spot severity occurred when fungicides were sprayed in the morning with dew present vs. dew displaced. However, they reported that chlorothalonil occasionally provided greater dollar spot control when applied in the morning after dew displacement or at noon to a dry canopy when compared with morning applications to dew-covered turf. The authors suggested improved chlorothalonil performance in the absence of dew may be possible because greater amounts of the fungicide adhere to the dry foliage and/or would be less likely to become diluted.
In a recent study, Pigati et al. (2010) concluded that early morning mowing before fungicide applications improved the performance of fungicides compared with plots mowed in the afternoon. Although some information exists for dollar spot control relative to dew removal strategies alone, or after fungicide applications, the impact of dew at the time of fungicide application is still not well understood.
The application of PGRs has become a conventional golf course turf management practice for the regulation of turfgrass growth; suppression of certain weeds; reduction in mowing frequency and clipping yield; and enhancement of turfgrass color, quality, and density (Watschke et al., 1992). TE, paclobutrazol, and flurprimidol are the most commonly used PGRs on golf courses in the United States. Two of these PGRs, flurprimidol and paclobutrazol, have been shown to inhibit growth of S. homoeocarpa to a greater extent than TE (Burpee et al., 1996). Field studies have shown that when used in combination with fungicides, flurprimidol and paclobrutrazol enhance dollar spot control (Fidanza et al., 2006; Putman and Kaminski, 2011; Stewart et al., 2007).
Results from research on the influence of TE on dollar spot severity are inconsistent. Field research findings have described neutral, beneficial, or negative effects of TE on fungicide performance for the control of dollar spot. Burpee et al. (1996) found that when TE was applied alone, it had no significant effect on dollar spot, but when applied in combination with chlorothalonil, iprodione, and propiconazole, it enhanced fungicide efficacy in 1 year of the 2-year study. Putman and Kaminski (2011) reported TE had no influence on dollar spot when applied in combination with fungicides. In the absence of fungicides, however, TE has been shown to significantly suppress dollar spot (Golembiewski and Danneberger, 1998; Putman and Kaminski, 2011). In another study conducted by Stewart et al. (2007), TE rarely influenced dollar spot severity or fungicide performance when applied before the onset of dollar spot symptoms. When applied after the onset of symptoms, turf recovery from dollar spot damage after applications of chlorothalonil and propiconazole was significantly delayed within TE-treated plots on some occasions. Stewart et al. (2007) suggested that diminished turf growth resulting from TE applications may reduce fungicide uptake and limit the suppressive effect of the active ingredient, thereby delaying recovery from the outbreak. Several authors have reported TE improved grass tolerance to abiotic stress and may stimulate non-fungistatic mechanisms that contribute to disease suppression (Burpee et al., 1996; Golembiewski and Danneberger, 1998; McCann and Huang, 2007; Xu and Huang, 2010; Zhang and Schmidt, 2000).
The objectives of this research were to: 1) evaluate the influence of dew removal at the time of fungicide application on dollar spot control; and 2) determine the effect of turfgrass regulation by TE on fungicide performance for dollar spot control.
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