Flumorph is a carboxylic acid amide (CAA) fungicide with high activity against oomycetes. However, evolution to CAAs from low resistance to high resistance has never been reported. This study investigated the basis of resistance evolution of flumorph in Phytophthora sojae. Total of 120 P. sojae isolates were collected and their sensitivity to flumorph was evaluated. Although no spontaneous resistance was found among the field isolates, adaptation on flumorph‐amended media resulted in the selection of five stable mutant types exhibiting varying degrees of resistance to CAAs. Type I, which exhibited the lowest resistance level, was obtained when the wild‐type isolate was exposed to a low concentration of flumorph, but no resistant mutants were obtained by direct exposure to higher concentrations. However, the more resistant types (Type II, III, IV and V) were obtained when Type I were exposed to higher concentrations of flumorph. Similar results were obtained when the entire screening process was repeated, which implied that evolution of resistance to flumorph in P. sojae could be a two‐step process, where high resistance phenotypes could develop gradually from low resistance ones. Further investigation into molecular mechanism strongly confirmed that evolution of isolates highly resistant to flumorph occurs in a stepwise process with Type I as intermediary, through accumulation of mutations in their target protein of CAAs (CesA3). Together, our findings indicate that application of low rates of flumorph in field could result in selection of low resistance Type I isolates, but that raising dosage to maintain comparable levels of control could elicit rapid evolution of more resistant Type II, III, IV and V isolates with stepwise accumulation of mutations in CesA3, which would render flumorph ineffective as a control method. Precautionary resistance management strategy should be implemented. The phenomenon described in the study could have broader biological significance.

中文翻译：

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大豆疫霉中 CesA3 突变的逐步积累导致对 CAA 杀菌剂的抗性增加

Flumorph 是一种羧酸酰胺 (CAA) 杀菌剂，对卵菌具有高活性。然而，从未报道过从低抗性到高抗性向 CAA 的演变。本研究探讨了大豆疫霉中氟吗啉抗性演化的基础。共 120 份酱油收集分离物并评估它们对氟吗啉的敏感性。尽管在田间分离株中未发现自发抗性，但对氟吗啉改良培养基的适应导致选择了五种对 CAA 表现出不同程度抗性的稳定突变类型。当野生型分离株暴露于低浓度的氟吗啉时获得了表现出最低抗性水平的 I 型，但直接暴露于较高浓度的氟吗啉没有获得抗性突变体。然而，当 I 型暴露于较高浓度的氟吗啉时，会获得更强的抗性类型（II、III、IV 和 V）。当重复整个筛选过程时获得了类似的结果，这表明大豆对氟吗啉的抗性进化可能是一个两步过程，其中高抗性表型可以从低抗性表型逐渐发展。对分子机制的进一步研究有力地证实，对氟吗啉具有高度抗性的分离株的进化发生在以 I 型为中介的逐步过程中，通过其 CAA 靶蛋白 (CesA3) 中突变的积累。总之，我们的研究结果表明，在田间施用低比例的氟吗啉可能会导致选择低抗性 I 型分离株，但提高剂量以维持可比较的控制水平可能会引发更具抗性的 II、III、IV 和 V 型的快速进化CesA3 突变逐步积累的分离株，这将使氟吗啉作为控制方法无效。应实施预防性耐药管理策略。