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

ABSTRACT 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-resista...