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

Corynespora leaf spot (CLS) caused by<i> C. cassiicola</i> 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 <i>C. cassiicola</i> 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 <i>C. cassiicola</i>, 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.