The weather is getting warmer and the world is slowly starting to open back up, but there is nobody more excited for golf season than our favorite turfgrass pathogen, dollar spot. As always there is no shortage of interest and fascination for dollar spot and its complex biology, resilience, and abilities for adaptation. As new information becomes available, agricultural chemical companies are at the forefront of developing and evaluating new active ingredients for control of this pesky disease. As a result, there are many new and exciting products now on the market that could provide valuable alternatives for controlling populations of dollar spot, particularly those that are less sensitive to certain classes of fungicides.
Chemical fungicides are often a necessary tool for adequately maintaining highly managed turf areas like golf course fairways, greens, and tee boxes. While cultural practices like balanced fertilization (research has shown that maintaining nitrogen fertility is particularly important for managing dollar spot), dethatching, lightweight rolling, adequate irrigation, and knocking down dews and guttation can be useful for reducing overall dollar spot damage in the field, these practices are frequently not sufficient for maintaining the superior turf quality that is expected on a golf course. To maintain optimal health and performance, repeated applications of multiple fungicides from different classes is crucial. The most effective fungicide classes for dollar spot control are the single-site mode of action (MOA) classes of fungicides like benzimidazoles, dicarboximides, demethylation inhibitors (DMI), and succinate dehydrogenase inhibitors (SDHI) as well as the multi-site MOA fungicides like chlorothalonil and fluazinam. However, many populations of dollar spot across the country are developing the capability of resisting, and therefore surviving, in the presence of many or almost all single-site MOA fungicides.
Some of the most important compounds used for control of fungal pathogens in both medicine and agriculture are within the site-specific and penetrant demethylation inhibitor (DMI) fungicide class. DMI fungicides work by blocking an enzyme in fungal pathogens known as cytochrome P450 sterol 14α-demethylase (CYP51), which is essential for making ergosterol, a vital component for fungal cell membranes that is necessary for cell growth. Until recently, it has been well understood that many phytopathogenic fungi, including C. homoeocarpa (the causal agent of dollar spot), exhibit “complete cross”-resistance among DMIs, in which resistance to one compound within the class means resistance to all others within that class. However, a newly released DMI active ingredient on the market, mefentrifluconazole (trade name: Maxtima®) has been shown to provide exceptional control of many turf diseases, including dollar spot, without the excessive plant growth regulation leading to phytotoxicity that can often be an issue in hot summer months with other DMI fungicides. But what does that mean for managing DMI-resistant populations of dollar spot and other phytopathogenic fungi?
Highly interested in understanding fungicide resistance in dollar spot, we set out to answer some of those questions. Following in vitro studies with dollar spot fungi grown on petri dishes in the lab, mefentrifluconazole was able to inhibit fungal growth by 50% at a much lower concentration (EC50) than that of two commonly used DMI fungicides, propiconazole and tebuconazole, suggesting a higher potency and control efficacy of this active ingredient at lower concentrations. Our data, which compared EC50 values of three DMI fungicides against two DMI-sensitive and two DMI-insensitive isolates of dollar spot, also shows that mefentrifluconazole could be useful for managing dollar spot populations that are less sensitive to DMIs. Where concentrations of 0.22 and 0.26 ppm for propiconazole and tebuconazole, respectively for were required to inhibit the growth of a highly DMI-insensitive isolate by 50%, mefentrifluconazole was able to inhibit this isolate by 50% with a concentration of only 0.01 ppm. This means that less of the fungicide can be used for adequate disease control, ultimately saving money by avoiding potential field failures or lack of longer residual control, while also reducing pressure for resistance development and reducing overall pesticide use.
Practitioners, however, should proceed with caution when using this product to control resistant populations of dollar spot, as over usage of this more potent active ingredient could still cause additional shifts toward DMI-insensitivity despite its current abilities for control since the pathogen is capable of evolving towards resistance. Less mefentrifluconazole was required to inhibit the growth of insensitive isolates by 50% as compared to propiconazole and tebuconazole treatments. However, EC50 values for mefentrifluconazole were higher on insensitive strains as compared to sensitive strains, suggesting confirmation of complete cross-DMI resistance in the dollar spot pathogen, albeit at a much lower level than that of propiconazole and tebuconazole.
Finding the right balance by applying fungicides at the recommended label rates and following the Fungicide Resistance Action Committee guidelines for resistance management is key for preventing the loss of important classes of fungicides like DMIs, particularly those with less phytotoxicity in the summer months. That means rotating DMIs with non-DMI fungicides (including multi-site MOA fungicides) as well as tank mixing with non-DMI fungicides. Predictive disease models should also be utilized to make informed decisions about fungicide applications. Under high disease pressure, (temperatures between 60-90 °F, warm days, cool nights, extended periods of leaf wetness), err on the side of caution when applying fungicides to fungicide resistant populations of dollar spot. Please remember to also make use of effective cultural practices, including biologicals, as much as possible to minimize overall dollar spot pressure.
Lastly, knowing where your dollar spot population stands in terms of resistance can be very useful for developing an effective management program. If you are interested in testing for fungicide resistance on your course, please contact us at the UMass Turf Pathology Lab for further information at email@example.com (Geunhwa Jung) or firstname.lastname@example.org (Michaela Elliott).
Submitted by: Dr. Geunhwa Jung and Michaela Elliott, Research Associate