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 mu g/ml with a mean of 0.14 mu g/ml. Examination of two DMI-exposed populations indicated an approximate increase of 6.5x in mean ED50 values. In vivo testing of two isolates with ED50 values of propiconazole at 0.15 and 0.90 mu g/ml indicated reduced control for the less sensitive isolate by propiconazole at rates <= 38 mu g/ml. It was determined that single discriminatory dose testing in vitro with propiconazole at 0.50 mu g/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 mu g/ml, respectively; for a DMI-exposed population, mean ED50 values were 3.8, 0.35, and 18 mu g/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.