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Rhizoctonia cerealis sensitivity to fludioxonil in China and analysis of laboratory fludioxonil-resistant mutants

Haiyan Sun: Jiangsu Academy of Agricultural Sciences

<div><em>Rhizoctonia cerealis</em> is a major soilborne fungal pathogen causing wheat sharp eyespot (WSE) in winter wheat in China. The phenylpyrrole fungicide fludioxonil effectively controls WSE, but the sensitivity and the potential for <em>R. cerealis</em> to develop resistance were little reported. We studied fludioxonil sensitivity in a total of ninety-two <em>R. cerealis</em> field isolates and evaluated the resistance risk in laboratory fludioxonil-resistant mutants. Our results showed that fludioxonil was effective against all field isolates. The EC<sub>50</sub> (effective concentration inhibiting 50% mycelial growth) values ranged from 0.009 to 0.064 mg l<sup>-1</sup> and were distributed unimodal around an average of 0.027 mg l<sup>-1</sup>. We further isolated fifteen <em>R. cerealis</em> laboratory mutants with low or high resistance levels to fludioxonil by exposing mycelia plugs on PDA plates containing fludioxonil. Fludioxonil-resistant mutants showed parental sensitivity to thifluzamide and tebuconazole, but resistance to iprodione, and slight resistance to jinggangmycin. Studies on fitness parameters of fludioxonil-resistant mutants revealed a fitness cost on mycelial growth and pathogenicity. Moreover, the mutants were more sensitive to osmotic stress (high sugar and high salt) than their wild-type isolates. Therefore, <em>R</em>. <em>cerealis</em> has a potentially low risk of developing resistance to fludioxonil.</div>