Dollar spot is the most problematic turfgrass disease on golf courses worldwide. More money is spent annually spraying fungicides to control this disease than any other. Unfortunately, dollar spot problems persist despite the availability of new grass varieties that have excellent resistance to dollar spot and years of research on its biology and control.
Fungicide resistance issues are higher on dollar spot than any other turfgrass disease, adding to the challenge of managing this pest. The benzimidazoles, dicarboximides, triazoles or DMIs, and even the newest class of chemistry, SDHIs, all have confirmed resistance issues.
Adding to the complex nature of dollar spot is the recent DNA work that has shown there is more diversity to the disease than originally believed. The genus of the pathogen that causes dollar spot has been renamed Clarieedia, and there are distinct species responsible for cool- and warm-season dollar spot symptoms. Two of these species infect both warm- and cool-season turfgrass. Isolates of another fungus causing dollar spot, Poculum, were identified in Florida and found on both symptomatic seashore paspalum and bermudagrass.
So, what does this all mean? The next time you see dollar spot, don’t assume that what you’re seeing is run-of-the-mill dollar spot. Take a step back and ask yourself a few questions. Is dollar spot becoming more problematic or occurring more often? Are my fungicide applications providing reduced control or shorter duration of prevention? You may have fungicide resistance, reduced sensitivity or maybe you have unintentionally managed for a different pathogenic species. Use a diagnostic lab, ideally in your state or region, for proper identification and recommendations before you make your next fungicide application.
Many of the fungicides labeled for dollar spot were likely tested on bentgrass and on only one species of the dollar spot pathogen in the northern U.S. Research is needed to better understand the differences in fungicide activity against these newly defined pathogenic species on different grasses and regions of the country. In the meantime, it doesn’t hurt to ask your supplier, local extension agent or your USGA agronomist for advice. Chances are, there are differences in fungicide sensitivity that can lead to inadvertently selecting for the most difficult to control species and strains.
Fungicides are great tools, but they will work better and longer if you know what you’re dealing with. They also work best when used in conjunction with cultural practices to alleviate environmental conditions that favor the pathogen. That said, regrassing to a resistant or less-susceptible variety and following sound cultural practices is the most cost-effective, long-term solution for managing dollar spot.