Rotating Fungicide Classes
During the growing seasons of 2002 and 2003, we sampled golf courses where superintendents suspected fungicide resistance in populations of Sclerotinia homoeocarpa. Their suspicions were based on less-than-satisfactory control from fungicides that provided acceptable results in the past. We collected hundreds of isolates of S. homoeocarpa, recorded sample locations, and conducted our laboratory assays to describe the sensitivity of each isolate to certain fungicide classes. Most isolates were not resistant–and we eventually attributed disappointing control to other factors that influence fungicide performance. In the cases where resistance was a serious issue, the resistant types were (almost always) collected from fairways rather than putting greens or tee boxes. This was an interesting observation with a logical explanation. The putting greens were treated with fungicides on a preventative basis (14-day application interval), and treatments often were tank mixes that included a multisite mode of action fungicide (e.g., chlorothalonil). Conversely, fairways were being treated on as-needed basis, in response to an outbreak of symptoms. Also, because symptoms were apparent, superintendents tended to select site-specific mode of action products to quell outbreaks and promote recovery. By delaying applications until after the outbreaks occurred, pathogen populations were high, and therefore more likely to include mutants that were less sensitive to the site-specific active ingredients. Over years of post-outbreak treatments with certain site-specific compounds, resistant types increased in the population to the point where fungicide performance suffered. The take home message? Avoid or delay the evolution of resistant strains by applying fungicides preventatively, rely on multisite fungicides (consider Daconil et al. and Secure) in the tank mix, and, as always, rotate among fungicide classes.