Serial passage through resistant and susceptible cucumber cultivars affects the virulence of Fusarium oxysporum f. sp. cucumerinum

Fusarium oxysporum f. sp. cucumerinum (Foc) is the causal pathogen of cucumber Fusarium wilt resulting in losses to cucumber production. To investigate the effects of the selective pressures of host plants on the virulence of Foc, a low virulence isolate, foc‐3b, was successively inoculated on resistant and susceptible cucumber cultivars for five generations. The virulence of the original isolate diverged; virulence was significantly strengthened after serial passage on the resistant cultivar and weakened on the susceptible plants (p ˂ .05). The expression of four virulence‐related genes of F. oxysporum, G‐protein α subunit gene fga1, sucrose nonfermenting 1 gene snf1, F‐box protein gene frp1, and Class V chitin synthase gene chsV, was quantified using real‐time PCR. All genes were significantly upregulated after serial passage on the resistant cultivar, compared to the original strain, and the expression of snf1 was downregulated in strains re‐isolated from the susceptible plants (p ˂ .05). A significant positive correlation was found between the expression levels of gene snf1, frp1, and chsV and disease severity of cucumber Fusarium wilt, suggesting these genes may impact virulence differentiation. This study will improve the management of cucumber Fusarium wilt and provide insight into the mechanisms underlying virulence of F. oxysporum.