Sensitivity Distributions of California Populations of Colletotrichum cereale to the DMI Fungicides Propiconazole, Myclobutanil, Tebuconazole, and Triadimefon.

The baseline sensitivity of a California population of Colletotrichum cereale (turfgrass anthracnose) to the sterol demethylation inhibitor (DMI) fungicide propiconazole was determined using an in vitro assay with known reproducibility. The 50% effective dose (ED₅₀) values for propiconazole for a nonexposed, baseline population ranged from 0.025 to 0.35 μg/ml with a mean of 0.14 μg/ml. Examination of two DMI-exposed populations indicated an approximate increase of 6.5× in mean ED₅₀ values. In vivo testing of two isolates with ED₅₀ values of propiconazole at 0.15 and 0.90 μg/ml indicated reduced control for the less sensitive isolate by propiconazole at rates ≤38 μg/ml. It was determined that single discriminatory dose testing in vitro with propiconazole at 0.50 μg/ml could differentiate sensitive and resistant isolates. Using this dose, six additional populations were tested and DMI-exposed populations were found to be three to nine times less sensitive compared with the baseline population. Two populations were assayed for sensitivity to myclobutanil, tebuconazole, and triadimefon. Mean ED₅₀ values for a nonexposed population were 0.72, 0.082, and 5.6 μg/ml, respectively; for a DMI-exposed population, mean ED₅₀ values were 3.8, 0.35, and 18 μg/ml, respectively. This work provides information on the development of DMI resistance in populations of C. cereale in California and methodologies for future resistance monitoring for this pathogen.