Reassessment of Fungicide Synergism for Control of Dollar Spot.

Experiments were designed to assess reports of synergism between propiconazole and other fungicides to control dollar spot in creeping bentgrass. In 2004 and 2006, two field experiments were conducted near Griffin, GA, and repeated near West Lafayette, IN. A third experiment was conducted at the Griffin site in 2007. In each experiment, replicated treatments of commercial formulations of propiconazole, triadimefon, iprodione, vinclozolin, and chlorothalonil were applied to plots of creeping bentgrass at the sublabel rates of 0.12, 0.38, 0.57, 0.38, and 2.29 kg a.i. ha^-1, respectively. In addition, each of the latter four fungicides was tank mixed with propiconazole at the rates given, and applied as treatments. Plots were treated once in experiments 1 and 3, and three times at a 21-day interval in experiment 2. Nontreated plots served as controls. Disease intensity was assessed visually by estimating the area of each plot that exhibited dollar spot symptoms. Synergism between tank-mixed fungicides was determined by comparing actual and expected values of the duration of disease control (experiment 1) or the percentage of disease control (experiments 2 and 3) in treated plots. No synergistic interactions were detected at Griffin or West Lafayette in experiments 1 or 2. In the first trial of experiment 3, synergism was observed between propiconazole and iprodione on one of five ratings dates and between propiconazole and vinclozolin on two of five rating dates. However, no synergistic interactions were detected in the second trial. Results suggest that there is a low probability for turfgrass managers to take advantage of fungicide synergism to control dollar spot with the products and rates tested.