Authors
Xiao Hong Lu, Department of Plant Pathology, China Agricultural University, Beijing 100193, and Department of Plant Pathology, Michigan State University, East Lansing 48824;
Mary K. Hausbeck, Department of Plant Pathology, Michigan State University;
Xi Li Liu, Department of Plant Pathology, China Agricultural University; and
Jianjun J. Hao, Department of Plant Pathology, Michigan State University
Abstract
Crown, root, and fruit rot caused by Phytophthora capsici is an increasing problem for vegetable growers in Michigan and the United States. The newly registered fungicide fluopicolide is effective to limit crop loss but the potential for P. capsici to develop resistance is not well known. A laboratory study assessed the risk of P. capsici developing resistance to fluopicolide. Baseline sensitivity to fluopicolide was determined using 126 P. capsici Michigan isolates. Values of effective concentrations for 50% inhibition of mycelial growth ranged from 0.08 to 0.24 μg/ml and were distributed as a unimodal curve, indicating that all isolates were sensitive to fluopicolide. Mutants resistant to fluopicolide were obtained from five isolates by screening zoospores on fluopicolide-amended (5 μg/ml) media at a mutation frequency above 1.0 × 10–7. The mutant isolates were clustered with either intermediate (resistance factor [RF] = 3.53 to 77.91) or high (RF = 2481.40 to 7034.79) resistance. Resistance was stable through 10 mycelial transfers on fungicide-free medium. All resistant mutants showed similar fitness in zoospore production, cyst germination, and virulence compared with their sensitive parents, with few exceptions. No cross-resistance was detected between fluopicolide and five other fungicides. There could be a moderately high risk of field populations of P. capsici developing resistance to fluopicolide, and populations should be monitored.
