Trifloxystrobin Resistance Mechanisms in <i>Fusarium oxysporum</i> f. sp. <i>melonis</i>

Melon Fusarium wilt (MFW) is a soilborne disease caused by the ascomycete fungus <i>Fusarium oxysporum</i> f. sp. <i>melonis</i> (<i>Fom</i>), 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 <i>F. oxysporum</i>. To date, potential resistance mechanisms to trifloxystrobin are poorly understood, so the current study evaluated four stable laboratory mutants of <i>Fom</i> with high levels of resistance. Compared with their parental isolates, the fitness of the resistant mutants was found to be dramatically increased, with a significant (<i>P</i> < 0.05) increase in mycelial growth and spore production and germination, as well as an increase in pathogenicity. Molecular analysis of the <i>FOMG_03348</i> gene, which encodes a putative mitochondrial cytochrome b protein in <i>Fom</i>, identified seven amino acid changes (T20I, G70S, S140P, S142A, Q144E, M162I, and S179R) that were conserved in the sequences of all the resistant mutants. In addition, significantly (<i>P</i> < 0.05) different patterns of <i>FOMG_03348</i> 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 with 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 <i>F. oxysporum</i>, as well as data that might improve fungicide strategies for more effective control of MFW in the field.