Researches On Dimethachlon Resistance In Sclerotinia Sclerotiorum From China

Sclerotinia sclerotiorum (Lib.) de Bary is one of the most important fungal phytopathogens worldwide. Sclerotinia stem rot in oilseed rape caused by this pathogen occurrs seriously in China. S. sclerotiorum still imposes serious limitations on oilseed rape production and quality. Application of fungicides is the principal tool for control of Sclerotinia diseases currently.Sensitivity to dimethachlon was detected in a total of2904field isolates of S.sclerotiorum collected in2011and2012from six provinces of China, with the method of minimum inhibitory concentrantion (MIC) and EC50. Results showed that3(1.12%),2(1.02%),13(3.78%),8(3.29%) and11(2.69%) dimethachlon-resistant isolates were detected from Hunan Province in2012, Shaanxi Province in2011and2012, and Heilongjiang Province in2011and2012, respectively, with resistance ratios of4.56～105.53,10.06～23.32,24.37～99.80,5.56～85.49and3.21～9.69, respectively. The relative susceptibility baseline of S. sclerotiorum to dimethachlon was established as0.2963±0.1505(SD) μg/mL (0.0138～0.6778μg/mL)Compared with field dimethachlon-sensitive isolates, dimethachlon-resistant isolates grew more slowly on PDA medium, sclerotia formation was delayed, and was less pathogenic to leaves of oilseed rape and more sensitive to osmotic pressures, indicating that field resistant isolates had less fitness to compete with sensitive isolates under natural field conditions.Cross-resistance studies showed that there was positive cross-resistance between dimethachlon and iprodione, procymidone, and N-phenyl carbamate fungicide diethofencarb. No cross-resistance was found between dimethachlon and carbendazim, tebuconazole, kresoxim-methyl, thiram, and boscalid. However, only one dimethachlon-resistant isolate (HLJ4) showed positive cross-resistance between dimethachlon and the phenylpyrrole fungicide fludioxonil.Activities of field dimethachlon-resistant isolates’s superoxide dismutase (SOD) and phenylalanine ammonia lyase (PAL) were higher than those of sensitive isolates (P＜0.01), but activities of dimethachlon-resistant isolates’s gultathione s-transferases (GST) had no differences (P>0.05) compared with those of sensitive isolates. Sequencing of the two-compoent histidine kinase (HK) gene demonstrated that of the5dimethachlon-resistant isolates, only one isolate (HLJ4) had two point mutations with T replaced by C at position694and G replaced by A at position1245. According to the deduced amino acids sequence, isoleucine was replaced by tryptophane at position231. Further research was needed to explore whether this mutation was the underlying molecular mechanism for resistance to dimethachlonin in isolate HLJ4. The second point mutation was nonsense according to deduced amino acids sequence.Mixture preparation of50.0%tebuconazole·dimethachlon wettable powder (WP, tebuconazole: dimetachlone=12:11) had high activity against S. sclerotiorum both in laboratory and in field conditions, and the field efficacy experiments showed that the control efficacies of the mixture at20.0～25.0g (a.i.)/667m2against S. sclerotiorum reached67.1～88.7%. In addition, the processed50.0%tebuconazole·dimethachlon WP was safe to oilseed rape crop.