Abstract
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.
Highlights
This study will improve the management of cucumber Fusarium wilt and provide insight into the mechanisms underlying virulence differentiation in Foc
In strains re-isolated from the resistant cucumbers, the relative expression of frp1 increased with increasing serial passage, a slight decrease was observed in the fourth generation
We found that the virulence of the tested strain was significantly strengthened after serial passage on the resistant cultivar and weakened on the susceptible plants, suggesting cultivar selection may play an important role in Fusarium virulence differentiation and rotation or mixed planting of both cultivars in continuous cucumber production may reduce or delay the incidence of Fusarium wilt
Summary
The gene expression levels of fga in most strains derived from the resistant cucumbers were significantly higher than those in foc-3b and strains from susceptible plants, especially in the first generation. TA B L E 2 Analysis of variance in disease index of cucumber Fusarium wilt caused by Foc strains serially passed through resistant and susceptible cultivars. After serial passages on the resistant cultivar, the gene expressions of snf in most strains were upregulated markedly compared to the original one and the strains through susceptible cucumbers. In strains re-isolated from the resistant cucumbers, the relative expression of frp increased with increasing serial passage, a slight decrease was observed in the fourth generation. ChsV expression levels increased in the first two FIGURE 2 Differential expression of four genes in Foc strains serially passaged through different cucumber cultivars using quantitative reverse transcriptase PCR.
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