Double mutations in succinate dehydrogenase are involved in SDHI resistance in Corynespora cassiicola

Corynespora leaf spot (CLS) caused by C. cassiicola has become one of the most important plant diseases in cucumber. At present, the main method to control CLS depends on fungicides, such as succinate dehydrogenase inhibitors (SDHIs). However, the incidence of SDHIs resistance in C. cassiicola populations has gradually increased in recent years due to the large-scale reuse of these fungicides and their single mode of action, leading to the failure to control Corynespora leaf spot. With increasing dosages and service times, even greater resistance to SDHIs caused by double mutations may occur in the field. In this paper, site-directed mutagenesis was performed on a single-point mutant strain to generate strains with double mutations in the sdh gene. All of the double mutations of succinate dehydrogenase were involved in SDHIs resistance in C. cassiicola, with different levels of SDHI resistance among the mutant strains. Under the selective pressure of SDHIs, a single mutation conferring moderate to low resistance may evolve into double mutations conferring higher resistance to SDHIs. Therefore, we should reduce the application of SDHIs used in controlling leaf spot in greenhouse-cultivated cucumber and recommend using other types of fungicides for controlling this disease.

由多主棒孢霉（Corynespora cassiicola，简称C. cassiicola）引发的棒孢叶斑病（Corynespora leaf spot, 简称CLS）现已成为黄瓜上最为重要的植物病害之一。当前防治该病害的主要手段依赖于琥珀酸脱氢酶抑制剂（succinate dehydrogenase inhibitors, SDHIs）类杀菌剂。然而近年来，受限于这类杀菌剂的大规模重复施用与单一的作用机制，多主棒孢霉种群对SDHIs的抗药性发生率逐步升高，导致棒孢叶斑病防治失效。随着施药剂量增加与使用时长延长，田间甚至可能出现携带双突变、对SDHIs抗性更强的菌株。本研究对单突变菌株实施定点诱变，成功获得了sdh基因携带双突变的菌株。所有琥珀酸脱氢酶的双突变位点均与多主棒孢霉的SDHIs抗药性相关，且不同突变菌株的SDHIs抗性水平存在显著差异。在SDHIs的选择压力下，可赋予中低水平抗性的单突变可进化为赋予更高水平SDHIs抗性的双突变。因此，我们应当减少温室栽培黄瓜中用于防治该叶斑病的SDHIs施用量，并推荐使用其他类型的杀菌剂来防控此病害。