Trifloxystrobin Resistance Mechanisms in Fusarium oxysporum f. sp. melonis

Melon Fusarium wilt (MFW) is a soil-borne disease caused by the ascomycete fungus Fusarium oxysporum f. sp. melonis (Fom), which causes serious damage throughout the entire growth cycle of its hosts. However, the novel strobilurin fungicide, trifloxystrobin, has been shown to exhibit high activity against F. oxysporum. To date, potential resistance mechanisms to trifloxystrobin are poorly understood, so the current study evaluated four stable laboratory mutants of Fom with high levels of resistance. Compared to their parental isolates, the fitness of the resistant mutants was found to be dramatically increased with a significant (p < 0.05) increase in mycelial growth, and spore production and germination, as well as an increase in pathogenicity. Molecular analysis of the FOMG_03348 gene, which encodes a putative mitochondrial cytochrome b protein in Fom, identified seven amino acid changes (T20I, G70S, S140P, S142A, Q144E, M162I, and 179R) that were conserved in the sequences of all the resistant mutants. In addition, significantly (p < 0.05) different patterns of FOMG_03348 expression were detected, which resulted in the down-regulation of the gene in both the absence and presence of trifloxystrobin in all of the mutants compared to their parental isolates. However, no evidence of cross-resistance was found between trifloxystrobin and the DMI fungicides flutriafol, prochloraz and tebuconazole. Furthermore, negative cross-resistance was observed with carbendazim and fluazinam, and only a moderate positive correlation with fludioxonil. These results provide new insight into potential trifloxystrobin resistance mechanisms in F. oxysporum, as well as data that might improve fungicide strategies for more effective control of MFW in the field.