Introduction
Dollar spot is a common and costly disease affecting golf course grass caused by fungi from the genus Clarireedia. Turf managers usually rely on fungicides to control this disease, but these chemicals have become less effective over time because of fungicide resistance. In this blog, we will explain fungicide resistance, the current state of resistance of dollar spot, and strategies to manage and reduce this issue.
What is Fungicide Resistance?
Fungicide resistance happens when some fungi in a population survive treatments used to kill them. Initially, when a fungicide is applied, it targets most fungi, but a few resistant ones survive. Over time, these surviving resistant fungi reproduce, and their population grows. As the same fungicide is used repeatedly, the resistant fungi become more common, making the treatment less effective. To prevent this, it’s essential to use different fungicides and other methods to control fungal diseases.
Here are key ideas to remember:
Genetic Variation: Within any fungal population, there are natural genetic differences. Some fungi might have mutations that make them less sensitive to a fungicide.
Selection Pressure: When a fungicide is applied, it kills the sensitive fungi but leaves behind those with resistance mutations.
Reproduction: The resistant fungi survive and reproduce, passing their resistance traits to their offspring.
Population Shift: Over time, the population becomes mostly resistant, making the fungicide less effective.
Current State of Resistance of Dollar Spot
Many Clarireedia species have developed resistance to common fungicides like benzimidazoles (e.g., thiophanate-methyl) and demethylation inhibitors (DMIs, e.g., propiconazole). This resistance makes it harder to control dollar spots using these fungicides (Frontiers).
Here is a list of resistances so far:
FRAC Code 1 - Benzimidazoles: This class, including thiophanate-methyl, exhibits qualitative resistance, where isolates are either resistant or non-resistant.
FRAC Code 2 - Dicarboximides: Resistance to this class has also been observed, contributing to the reduced efficacy of these fungicides.
FRAC Code 3 - Demethylation Inhibitors (DMIs): Resistance to DMIs, such as fenarimol, metconazole, myclobutanil, propiconazole, triadimefon, and triticonazole, has been reported since the early 1990s. DMI resistance is often quantitative, resulting in partial resistance rather than complete insensitivity.
FRAC Code 7 - Succinate-dehydrogenase Inhibitors (SDHIs): Reports have indicated resistance to next-generation SDHI fungicides, with various mutations leading to differential sensitivities to the active ingredients.
This list is from information from Pennsylvania State University, University of Massachusetts Amherst, and North Carolina State University.
Updated FRAC codes can be found here: https://www.frac.info/knowledge-database/knowledge-database
Strategies to Manage and Mitigate Fungicide Resistance
Given the increasing resistance in dollar spot pathogens, it is crucial to adopt integrated disease management strategies. Here are some effective approaches:
1. Fungicide Rotation and Mixtures
Rotate Fungicides: Use different types of fungicides to reduce the pressure on any one type. This helps delay resistance. However, it is important to note to use fungicides with different modes of action (different FRAC codes).
Tank Mixing: Mix fungicides with different action methods to hit the fungus from multiple angles, reducing the chance of resistance.
2. Use of Microbial Inoculants to Mitigate Fungicide Resistance
Incorporate Biofungicides:
Bacteria like Bacillus subtilis QST713 have shown promise in reducing disease severity and can be used in rotation with synthetic fungicides to enhance overall disease management (Frontiers).
Other bacterial pesticides were shown to have limited action (Agronomy Journal):
B. amyloliquefaciens strain D747 is a broad range biological pesticide for foliar plant pathogens, but not specifically for turfgrass
Pseudomonas chlororaphis strain AFS009 is a broad range biological pesticide for foliar plant pathogens
Streptomyces lydicus strain WYEC 108 is a broad range biological pesticide for foliar plant pathogens, but not specifically for turfgrass
Incorporate Microbial Inoculants:
In our University of California Riverside Trials, combining En-Turf™ with Syngenta's Heritage TL fungicide resulted in a 27% reduction in summer patch compared to the control. En-Turf, combined with 50% fungicide, achieved the same reduction as using 100% fungicide alone.
En-Turf combined with BASF’s Maxtima at 50% of its standard rate. This led to a 35% reduction in Anthracnose disease cover, while Maxtima by itself at the full rate was observed to lower the disease by 22%
En-Turf combined with Syngenta's Briskway at 50% of its standard rate. This led to a 75% reduction in Anthracnose disease cover, while Briskway by itself at 50% of full rate was observed to lower the disease by 47%
“Fewer fungicide applications would, obviously, save golf courses money. That’s not a small thing, Kaminski explains. If managers can learn to assess the fungicide resistance of dollar spot and choose the right chemicals, he estimates they might avoid, on average, three fungicide applications a year.” - Penn State University News
3. Cultural Practices
Optimize Turf Health: Implementing proper cultural practices such as using resistant cultivars, adequate fertilization, proper irrigation timing, and thatch removal can decrease your risk of an outbreak.
Reduce Stress: Minimizing turf stress through good management practices can make the turf less susceptible to dollar spots and reduce the need for fungicides.
4. Regular Monitoring and Adaptation
Disease Monitoring: Continuously monitor turf for signs of disease and resistance development. This allows for timely adjustments to management strategies and fungicide applications.
Responsive Management: Be prepared to adjust fungicide programs based on monitoring results and emerging resistance patterns.
Conclusion
Fungicide resistance in dollar spot pathogens is a significant challenge for turf managers, but understanding how resistance develops and adopting integrated management strategies can help mitigate its impact. By rotating fungicides, incorporating alternatives, optimizing cultural practices, and continuously monitoring turf health, turf managers can maintain effective control of dollar spots and ensure their turf's long-term health and quality.
References
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