Biological characteristics of Corynespora cassiicola with different types of resistance to boscalid

Corynespora leaf spot caused by Corynespora cassiicola has become an important disease affecting cucumber. To provide the basis of biological research for exploring different resistance mechanisms of C. cassiicola to boscalid, the biological characteristics and environmental fitness of C. cassiicola with different types of resistance to boscalid were determined in this study. A total of 24 strains of C. cassiicola collected from different regions with different resistant types to boscalid were randomly selected. Seven types of resistant mutants cross-resistance to several different succinate dehydrogenase inhibitors (SDHIs) were analyzed and whether the mutants could inherite stably after cultivated nine generations on PDA plates without any fungicides selection pressure were determined. Meanwhile, the biological characteristics of resistant mutants under different carbon and nitrogen sources, temperature and other conditions were also determined to evaluate the fitness of resistant mutants under different environmental conditions. The results indicated that only SdhB-H278Y and SdhB-H278R mutants showed negative cross-resistance between boscalid and fluopyram, while drug resistance in all strains of the different mutation types could be inherited stably. Pathogenicity of different mutation types strains were discrepant, and the SdhD-D95E mutants had the strongest pathogenicity. The most favorable carbon source for the growth of all resistant mutants was maltose, and no discrepancy was showed in the effects of different nitrogen sources. The most optimal growth temperature range of each mutant strain was 25-30 ℃, while SdhD-D95E mutants grew faster than other mutants at temperatures greater than 30 ℃. In the study of heat resistance, the resistant mutants were not able to survive after treated at 65 ℃ for 45 min. At the same time, the mutant could grow normally under the condition of 60 ℃, while the sensitive strain could not. The mycelium growth rate of SdhD-D95E mutants were faster than that of other mutant strains, while SdhB-H278Y mutants were slower than that of other mutant strains under treatment with various concentrations of NaCl. Glucose was important to the growth of SdhB-H278R mutants. The results showed that the biological characteristics and fitness of C. cassiicola with different boscalid-resistance were discrepant. The adaptability of SdhD-D95E mutants was improved, indicating that the mutant had stronger competitiveness in the field and was easy to form a dominant population.