Pathogenicity Stimulation of Sclerotinia sclerotiorum by Subtoxic Doses of Carbendazim

Sclerotinia sclerotiorum is a devastating ascomycete fungus capable of infecting more than 400 species of plants worldwide. Carbendazim has been a principal fungicide for control of this pathogen and high levels of carbendazim resistance have been reported in eastern China. In this study, stimulatory effect of subtoxic doses of carbendazim on pathogenicity of S. sclerotiorum was investigated. All seven field resistant isolates with EC50 values greater than 1,000 μg/ml exhibited stimulated pathogenicity toward detached leaves of rapeseed at subtoxic concentrations of carbendazim. Detailed studies on pathogenicity of two resistant isolates AH-17 and LJ-86 toward potted rapeseed plants and detached leaves demonstrated that carbendazim at 0.2 to 5 μg/ml could consistently stimulate significantly higher (P < 0.05) pathogenicity than the control. On potted rapeseed plants, the percent stimulations on pathogenicity ranged from 18.8 to 22.0% for isolate AH-17 and from 15.1 to 23.2% for isolate LJ-86. On detached leaves of rapeseed, the percent stimulations ranged from 18.7 to 31.29% for isolate AH-17 and from 16.7 to 24.3% for isolate LJ-86. Studies on stimulation mechanism indicated that secretion of oxalic acid and tolerance to oxidative stresses H2O2 and paraquat after exposed to subtoxic doses of carbendazim did not change significantly. These results have profound implications for judicious application of fungicides and sustainable management of fungicide resistance.