Sensitivity of Fusarium graminearum to fungicide JS']JS399-19:: In vitro determination of baseline sensitivity and the risk of developing fungicide resistance

The baseline sensitivity of Fusarium graminearum Schwade [teleomorph = Gibberello zeae (Schweinitz) Petch] to the fungicide JS399-19 (development code no.) [2-cyano-3-amino-3-phenylacrylic acetate] and the assessment of risk to JS399-19 resistance in vitro are presented. The mean EC50 values for JS399-19 inhibiting mycelial growth of three populations of wild-type F graminearum isolates were 0.102 +/- 0.048, 0.113 +/- 0.035 and 0.110 +/- 0.036 mu g ml(-1), respectively. Through UV irradiation and selection for resistance to the Fungicide, we obtained a total of 76 resistant mutants derived from five wild-type isolates of F graminearum with an average frequency of 1.71 x 10(-7)% and 3.5%, respectively. These mutants could be divided into three categories of resistant phenotypes with low (LR), moderate (MR) and high (HR) level of resistance, determined by the EC50 values of 1.5-15.0 mu g ml(-1), 15.1-75.0 mu g ml(-1) and more than 75.0 mu g ml(-1), respectively. There was no positive crossresistancebetween JS399-19 and fungicides belonging to other chemical classes, such as benzimidazoles, ergosterol biosynthesis inhibitors and strobilurins, suggesting that JS399-19 presumably has a new biochemical mode of action. Although the resistant mutants appeared to have comparable pathogenicity to their wild-type parental isolates, they showed decreased rnycelial growth on potato-sucrose-agar plates and decreased sporulation capacity in mung bean broth. Nevertheless, most of the resistant mutants possessed fitness levels comparable to their parents and had MR or HR levels of resistance. As these Studies yielded a high frequency of laboratory resistance in F. graminearum, appropriate precautions against resistance development in natural populations should be taken into account.