Discriminatory dose determination for DMI fungicides for Clarireedia jacksonii and mycelial growth variation across active ingredients

In addition to cultural practices, dollar spot disease management requires frequent application of fungicides. Demethylation inhibitor (DMIs) fungicides are applied repeatedly to manage dollar spot in turfgrass systems. This facilitates reduction in DMI fungicide sensitivity among Clarireedia spp. populations. The objectives of this study were to (1) identify EC₅₀ and relative mycelial growth (RMG) values for Clarireedia spp. across nine commercially available DMI fungicides, (2) determine the discriminatory dose for each of these fungicides, and (3) elucidate differences among isolates exhibiting varying degrees of sensitivity to propiconazole across all commercially available DMI fungicides. Discriminatory doses were determined to be 0.01 for propiconazole and prothioconazole, 0.1 for flutriafol and tebuconazole, and 1.0 µg active ingredient (a.i.) mL⁻¹ potato dextrose agar for triadimefon, mefentrifluconazole, metconazole, triticonazole, and myclobutanil. Mefentrifluconazole completely inhibited mycelial growth of sensitive isolates. There were varying levels of mycelia suppression across highly insensitive isolates (RMG > 80%). Across highly insensitive isolates, prothioconazole provided the least suppression of growth, while mefentrifluconazole and myclobutanil provided the highest suppression of mycelial growth. However, none of the fungicides completely inhibited mycelial growth of the moderately sensitive and highly insensitive isolates. Results suggest variable suppression may exist among commercially available DMIs among sensitive and insensitive isolates of Clarireedia jacksonii.