Abstract
Fusarium oxysporum f. sp. niveum (FON) is the causal agent of Fusarium wilt in watermelon, an international growth-limiting pathogen of watermelon cultivation. A single demethylation inhibitor (DMI) fungicide, prothioconazole, is registered to control this pathogen, so the risk of resistance arising in the field is high. To determine and predict the mechanism by which FON could develop resistance to prothioconazole, FON isolates were mutagenized using UV irradiation and subsequent fungicide exposure to create artificially resistant mutants. Isolates were then put into three groups based on the EC50 values: sensitive, intermediately resistant, and highly resistant. The mean EC50 values were 4.98 µg/mL for the sensitive, 31.77 µg/mL for the intermediately resistant, and 108.33 µg/mL for the highly resistant isolates. Isolates were then sequenced and analyzed for differences in both the coding and promoter regions. Two mutations were found that conferred amino acid changes in the target gene, CYP51A, in both intermediately and highly resistant mutants. An expression analysis for the gene CYP51A also showed a significant increase in the expression of the highly resistant mutants compared to the sensitive controls. In this study, we were able to identify two potential mechanisms of resistance to the DMI fungicide prothioconazole in FON isolates: gene overexpression and multiple point mutations. This research should expedite growers’ and researchers’ ability to detect and manage fungicide-resistant phytopathogens.
Highlights
Fusarium wilt of watermelon, caused by the ascomycete fungus Fusarium oxysporum f. sp. niveum (FON), is a leading factor limiting watermelon production worldwide [1,2,3,4,5].Symptoms include single vine wilting, tip necrosis, dieback, and eventual plant death
Nine FON mutants were generated using the UV irradiation method described in the methods section that showed resistance to prothioconazole
Resistant isolates were separated into two groups, one as intermediately resistant (IR) and the other as highly resistant (HR)
Summary
Symptoms include single vine wilting, tip necrosis, dieback, and eventual plant death. This widespread pathogen is soil-borne and produces three different spore types: microconidia, macroconidia, and chlamydospores [3,6]. Symptoms are caused by the host defense response to develop tyloses which attempt to block the vascular spread of the pathogen. Developing tyloses clog up the passage of water and nutrients within the plant, causing loss of turgor pressure and wilting [7,8]. While micro- and macroconidia cause inseason spread of FON and hyphal structures can survive overwinter, chlamydospores can survive in soils for up to 10 years and are resistant to current management measures [9,10,11]
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