Risk assessment of Phytophthora capsici resistant to fluopicolide
X. LU (1), M. Hausbeck (2), X. Liu (1), J. J. Hao (2)
(1) China Agricultural University, Beijing, PRC PEOPLES REP OF CHINA; (2) Michigan State University, East Lansing, MI, U.S.A.
Phytopathology 101:S110

Fluopicolide is a systemic fungicide affecting oomycetes including Phytophthora capsici. A laboratory study was conducted to assess the risk of fluopicolide resistance in P. capsici. Baseline sensitivity to fluopicolide was determined using 126 P. capsici isolates from Michigan, U.S.A. All isolates were sensitive to fluopicolide and values of effective concentrations for 50% inhibition of mycelial growth (EC50) ranged from 0.0847 to 0.2380 µg/ml, with a mean of 0.1572 µg/ml in unimodal distribution. Resistant mutants were obtained at a mutation frequency above 1.0 × 10^–7 from five P. capsici isolates by screening zoospores on fluopicolide-amended (5 µg/ml) agar plates. The mutants showed either intermediate (resistance factors between 3.53 to 92.63) or high resistance (resistance factors between 2245.09 to 7034.79) to fluopicolide. The fluopicolide resistance of the mutants was stable through 10 mycelial transfers on fungicide-free medium. All resistant isolates exhibited an equal overall level of fitness (zoospore production, cyst germination, and virulence on zucchini fruit or pepper seedlings) compared with their sensitive parental isolates, with few exceptions. No cross-resistance was detected between fluopicolide and five other fungicides, including cyazofamid, mandipropamid, mefenoxam, zoxamide and azoxystrobin. Based on these results, the potential for P. capsici populations to develop resistance to fluopicolide in the field and the resistance risk may be moderately high.