| Cucumber downy mildew caused by the oomycete pathogen Pseudoperonosporacubensis is a devastating disease on cucumber. It spreads quickly and leads to vast lossesof yield and quality in the production of cucumber. So far, the chemical fungicide is stillthe major measures to control the disease. Mandipropamid belonging to the carboxylic acidamide class is a novel fungicide used for controlling the oomycete diseases; there was noreport about resistance risk of P. cubensis to mandipropamid. In this study the sensitivity ofP. cubensis to mandipropamid was determined, the baseline sensitivity to mandipropamidof P. cubensis was established, the resistance risk was assessed and the resistancemechanism of P. cubensis to mandipropamid with series of methods was clarified. Themain results are summarized as follows:1.69P. cubensis strains were collected from the areas in Hebei and Shandongprovince where no CAAs fungicides had been used before. The sensitivity tomandipropamid of the69wild-type strains was determined by leaf disc floating test. TheEC50ranged from0.096to0.695μg/mL, and the mean EC50value was (0.358±0.144)μg/mL. The frequency of strains with different sensitivity to mandipropamid distributed asa unimodal curve, there was no sub-group with sensitivity declined, so the mean EC50(0.358±0.144) μg/mL could be regarded as the baseline sensitivity to mandipropamid of P.cubensis for field resistance monitoring.2. P. cubensis isolates were collected from the areas in Hebei and Shandong provincefrom2011to2013. The sensitivity to mandipropamid of these P. cubensis isolates weretested by leaf disc floating method. The results showed that the main isolates weresensitive isolates, there were a small amount of low and medium resistance isolates, Nohigh resistance isolate was detected. According to the data, we could see that the sensitivitychanged small to mandipropamid of these P. cubensis isolates.3. UV mutagensis test and fungicide adaptation test were applied to acquire resistantmutants of P. cubensis to mandipropamid. However, only six mutants of P. cubensisresistant to mandipropamid were obtained by UV mutagensis test from11sensitive isolates.The resistance factors of these mutants ranged from5.74to22.96fold, and the mutationfrequency was1.09×10-7. The fitness of six mandipropamid-resistant mutants wassignificantly lower than that of their respective parental strains, and the resistance could not be inherited stably. No cross resistance relationship existed between mandipropamidand metalaxyl, azoxystrobin, cymoxanil or fluopicolide, but the cross resistance wasexisted between mandipropamid and dimethomorph.4. CTAB method was used for extracting the DNA of P. cubensis strains used in thisstudy and sequencing analysis software DNAMAN was used for sequence alignmenting.The results showed that only the resistant strain KB-1-9UVshowed a mutation in the CesA3gene, at position940of its amino acid sequence, leading to an amino acid exchange fromvaline (GTT code) to alanine (GCT code), but the resistance of strain KB-1-9UVdisappeared after reproducting10generations and the mutation recovered, Therefore themutation of this site was unstable. Other mutant strains and moderate resistant strainscollected from the field had no genetic mutations in CesA3gene.5. It could conclude from the results of this study that P. cubensis had a low tomedium risk of developing resistance to mandipropamid. |
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