RESUMO
Previous studies have demonstrated that subtoxic doses of carbendazim have a stimulatory effect on pathogenicity of Sclerotinia sclerotiorum on rapeseed plants. The present study focused on the time-course profile of the stimulatory effect and its relevance to stimulation mechanisms. At 12 h postinoculation (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 nontreated control. At 18 HPI, carbendazim stimulation on pathogenicity was more obvious than at 12 HPI. Study with scanning electron microscopy demonstrated that no discernable differences in the development of disease symptoms could be detected at 8 HPI. However, at 12 HPI, necrotic symptoms of the epidermal cells were apparent only for leaves sprayed with carbendazim. These results indicated that stimulations on pathogenicity occurred in the first 12 h, implying that direct stimulation rather than overcompensation to the disruption of homeostasis was likely to be the underlying mechanism for pathogenicity stimulation. Greenhouse experiments showed that 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 stimulation action eventually disappeared for inoculations at 14 DAA. Mycelia grown on potato dextrose agar (PDA) supplemented with carbendazim at 400 µg/ml were more pathogenic than the nontreated control. However, after additional growth of the mycelia on fungicide-free PDA for 2 days, the stimulatory effect disappeared completely, indicating that carbendazim was indispensable for pathogenicity stimulations. Studies on biochemical mechanisms indicated that cell-wall-degrading enzymes such as cellulase, pectinase, and polygalacturonase were not involved in pathogenicity stimulations. These results will advance our understanding of the nature and mechanisms of fungicide stimulation on fungal pathogenicity and, thus, are valuable for judicious applications of fungicides.
RESUMO
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.
