Strategies for the management of fungicide-resistant Rutstroemia floccosum (syn. Sclerotinia homoeocarpa), the causal organism of dollar spot

Dollar spot is an important disease of turfgrasses worldwide. The pathogen, Rutstroemia floccosum, has developed resistance to three classes of systemic fungicides: the benzimidazoles, the dicarboximides, and the demethylation inhibitors (DMI). Two multiyear studies assessed changes in DMI sensitivities over time using DMI and non-DMI fungicides at different rates applied alone, in alternation, or in combination with each other.; The first experiment involved a DMI-resistant population of R. floccosum resident to a mixture of creeping bentgrass and annual bluegrass maintained as a golf-course fairway. Isolates taken at five time points were grown into pure culture and then assayed using relative comparisons of their radial growth on PDA and PDA amended with 2 mug ml-1 triadimefon (DMI). A similar experiment was conducted on a DMI-sensitive population of R. floccosum from another site. In both studies, the pathogen's resistance to DMI fungicides increased with all treatments that involved exposures to DMI fungicides. A positive relationship was shown between the number of DMI-fungicide applications and the rates of increase in DMI resistance. An AFLP analysis of a selection of DMI-resistant and -sensitive isolates failed to distinguish differences among those isolates.; A final investigation was conducted at the DMI-resistant R. floccosum site above where unsatisfactory dollar-spot control had occurred with a first-time use of boscalid, a new dollar-spot fungicide of the carboximide class. In a field experiment, significant numbers of dollar spots appeared at three days after treatment (DAT) with boscalid compared to a treatment with chlorothalonil only. The dollar spots had disappeared at 8 DAT. In a second experiment, the dollar spots began appearing at 4 DAT and had disappeared by 14 DAT. The number of dollar spots in the boscalid treatment was significantly greater than the chlorothalonil treatment at 9 and 12 DAT. Isolates were collected from the transient dollar-spots during the second experiment and found to have significantly greater in vitro resistance to boscalid compared to isolates of five different strains collected in other locations in Michigan.