Dollar spot response across DMI fungicides
Managing dollar spot disease involves regular applications of demethylation inhibitor (DMI) fungicides on golf course turfgrass. However, repeated applications can reduce dollar spot sensitivity to these DMI fungicides.
Research conducted by John Kaminski, Ph.D., and graduate student Maureen Kahiu evaluated the effective concentration (EC50) and relative mycelial growth (RMG) values for dollar spot species across nine commercially available DMI fungicides, determined the optimal concentration (i.e., discriminatory dose) for each fungicide and determined differences among isolates that show varying sensitivity to propiconazole across all commercially available DMI fungicides.
The determined doses (in micorgrams per milliliter or parts per million) needed to distinguish between responses were .01 for propiconazole and prothioconazole, .1 for flutriafol and tebuconazole and 1.0 micrograms per milliliter of active ingredient on potato dextrose agar for triadimefon, mefentrifluconazole, metconazole, triticonazole and myclobutanil. Mefentrifluconazole entirely stopped the growth of sensitive fungus strains.
There were varying levels of fungal growth suppression among highly resistant isolates (resistance management group > 80 percent). Among these resistant isolates, prothioconazole was the least effective at inhibiting growth, whereas mefentrifluconazole and myclobutanil were the most effective.
The sensitive baseline population showed significant variation in fungal growth when tested with nine different fungicides. The relative growth of the fungus among sensitive isolates and across all fungicides varied from 0 to 62.5 percent in the first experimental run and from 0 to 58.7 percent in the second run.
In both experimental runs, mefentrifluconazole and myclobutanil resulted in the least mycelial growth across isolates of all sensitivity. Triticonazole had similar RMG when screened across sensitive and moderately sensitive isolates. Prothioconazole and triadimefon generally resulted in the highest RMG across all isolates.
When tested with moderately sensitive populations, RMG values ranged from 15.7 to 59.1 percent in the first experimental run and from 5.6 to 59.3 percent in the second. Fungal growth steadily increased as the populations moved from moderately to highly insensitive.
These findings suggest that the effectiveness of commercially available DMIs varies among both sensitive and resistant strains of dollar spot (Clarireedia jacksonii).
Reference
Adapted from Kahiu, M.M., and Kaminski, J.E. 2025. Discriminatory dose determination for DMI fungicides for Clarireedia jacksonii and mycelial growth variation across active ingredients. Crop Science. 2025;65:e21418. https://doi.org/10.1002/csc2.21418
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