Occurrence and Distribution of QoI-Resistant Isolates of Colletotrichum cereale from Annual Bluegrass in California.

Turfgrass anthracnose, caused by Colletotrichum cereale (ex. Colletotrichum graminicola), is an important disease of turf used on golf course putting greens. Recent management of the disease has become increasingly difficult, partly due to the possible development of practical resistance to the QoI fungicides. In all, 558 single-conidia isolates of C. cereale were collected from 10 California golf courses, 8 of which had been exposed to QoI fungicides and 2 where no fungicides had been used. Isolates were tested using a mycelial expansion assay on azoxystrobinamended media. For the two nonexposed populations, in vitro 50% effective dose (ED₅₀) values ranged from 0.0060 to 0.089 μg/ml. All isolates from the exposed populations could not be fully inhibited by doses of azoxystrobin as high as 8.0 μg/ml. A subset of these isolates were tested in vitro with the QoI fungicides pyraclostrobin and trifloxystrobin and found to be similar in response, indicating that these isolates were fully cross-resistant to all three fungicides. In greenhouse pot experiments, three isolates nonresponsive to QoI fungicides in vitro were not controlled by label rates of the fungicides. Spore germination assays also were examined; for 10 isolates identified as sensitive by mycelial expansion assays, ED₅₀ values for axoystrobin ranged from 0.0040 to 0.0047 μg/ml; for 25 isolates identified as QoI-resistant, 93 to 100% of the conidia germinated at azoxystrobin concentrations as high as 8.0 μg/ml relative to the nonamended check treatments. Mitochondrial cytochrome b genes from a subset of 15 isolates (12 resistant and 3 sensitive) were partially cloned and sequenced; all resistant isolates had an alanine substitution that corresponded to position 143 of the gene product. These results indicate that QoI resistance is present in California populations of C. cereale and is contributing to the difficulty in controlling this disease.