Stimulatory Effect Of Low Doses Of Carbendazim On Sclerotinia Sclerotiorum

Sclerotinia sclerotiorum(Lib.) de Bary is a necrotrophic plant pathogen notorious for its worldwide distributions. This pathogen can infect more than 400 species of plants, and fungicide is still the mainstay for control of S. sclerotiorum. Carbendazim has been extensively used to control S. sclerotiorum for more than 3 decades and resistance to carbendazim has become serious since the late 1990 s in eastern China. The present study used two carbendazim resistant isolates with EC50>1000 μg/m L to determine pathogenicity stimulation of S. sclerotiorum by low doses of carbendazim.Spraying carbendazim at 0.2 to 5 μg/m L had significant(P<0.05) stimulatory effects on pathogenicity of S. sclerotiorum. On detached leaves of rapeseed, the percent stimulation on pathogenicity ranged from 18.7 to 31.3% for isolate AH-17 and from 16.7 to 24.3% for isolate LJ-86. On potted rapeseed plants sprayed with carbendazim at 0.2 to 5 μg/ml, the percent stimulation on pathogenicity ranged from 18.8 to 22.0% for isolate AH-17 and from 15.1 to 23.2% for isolate LJ-86. Mycelia grown on potato dextrose agar(PDA) supplemented with carbendazim at 400 μg/ml were more pathogenic than the non-treated control, and the percent stimulation on pathogenicity was 19.7% for isolate AH-17. However, after additional growing the mycelia on fungicide-free PDA for 2 days, the stimulatory effects disappeared completely. Spraying carbendazim at 400 μg/ml on potted rapeseed plants had statistically significant(P<0.05) stimulations on pathogenicity for inoculations at 1, 3, 5, and 7 days after application(DAA). The stimulatory effects eventually disappeared for inoculations at 14 DAA.The stimulatory effect of low doses of carbendazim on pathogenicity of S. sclerotiorum was a direct stimulation. Twelve hours post inoculation(HPI), initial necrotic lesions were visible only for rapeseed leaves treated with carbendazim at 0.2 and 1 μg/ml, whereas no disease symptoms were observed for the non-treated control. At 18 HPI, stimulation on pathogenicity was more obvious than at 12 HPI. Studies with scanning electron microscopy demonstrated that no discernable differences in the development of disease symptoms could be detected at 8 HPI. At 12 HPI, necrotic symptoms of the epidermal cells were apparent only for leaves sprayed with carbendazim.Studies on biochemical mechanisms for pathogenicity stimulation demonstrated that secretion of oxalic acid, tolerance to oxidative stresses H2O2 and paraquat, and activities of cell wall-degrading enzymes including cellulase, pectinase, and polygalacturonase did not change significantly(P>0.05) after exposure to subtoxic doses of carbendazim.