Pharmacological Characteristics and Efficacy of Fluazinam Against Corynespora cassiicola, Causing Cucumber Target Spot in Greenhouses

Abstract

Cucumber target spot, caused by Corynespora cassiicola is a devastative fungal disease in greenhouses in China. Lack of resistant cultivars and unscientific use of fungicides aggravated the difficulty to manage this disease. In recent years, resistance of C. cassiicola to benzimidazoles, quinone outside inhibitors (QoIs) and succinate dehydrogenase inhibitors (SDHIs) has occurred in China. Here, we tested the fluazinam sensitivity distribution of seventy-nine C. cassiicola isolates from different provinces in China based on mycelial growth inhibition. The EC50 values of fluazinam ranged from 0.1002 to 0.3129 µg/mL with a mean of 0.2136 ± 0.0495 µg/mL and the sensitivity frequency was normally distributed (P = 0.2083, Shapiro-Wilk test). Meanwhile, the EC50 values for spore germination inhibition ranged from 0.0992 to 0.2278 µg/mL with a mean of 0.1499 ± 0.0504 µg/mL. This indicated that fluazinam exhibited an excellent in vitro fungicidal activity on both mycelial growth and spore germination. In addition, fluazinam also exhibits a good in planta control efficacy on detached cucumber leaves in the protective and curative assays. Moreover, the biological and physiological characteristics of C. cassiicola as affected by fluazinam were determined. Fluazinam not only significantly inhibited respiration and ATP production, but also caused the increase of cell membrane permeability and the dysfunctions of cellular homeostasis. Interestingly, we found that fluazinam specially damaged vacuole structures, causing the re-distribution of vacuole substances. Taken together, our findings not only provide essential references for resistance management of C. cassiicola, but also provide interesting insights for further revealing the action mode of fluazinam against plant pathogens.