Baseline Sensitivity Of Pseudoperonospora Cubensis And Alternaria Solani To Azoxystrobin And Its Resistance Risk Assesment

Azoxystrobin was the first commercial strobilurin fungicide developed by Zeneca company with excellent fungicidal activity, broad anti-fungi spectrum and good safety to environment. The fungicidal activity of strobilurins stems from the fact that they can specifically inhibit the complex III in mitochondrial respiration of fungi. However, the resistance to strobilurins occurred just two years after the fungicides were introduced into the market. The objectives of this study were to establish the baseline sensitivity of Pseudoperonospora cubensis and Alternaia solani to azoxystrobin and to assess the resistance risk of P. cubensis to azoxystrobin.An excised leaf disc-floating test system was developed for determining the sensitivity of P. cubensis, cause agent of cucumber downy mildew, to azoxystrobin. Results showed that this sensitivity test method yielded reproducible results. Then, this method was applied to set up the sensitivity baseline of P. cubensis collected from Nanjing, Huaiyin and Donghai regions in Jiangsu province to azoxystrobin, where QoI and QiI fungicide never has been used. The median effective concentration(ECso) controlling cucumber downy mildew of azoxystrobin ranged from 0.00008 g/mL to 0.0484 g/mL among 83 isolates, with an average EC50 0.0062 g/mL. The EC50 of azoxystrobin and kresoxim-methyl against isolate N10 was 0.0140 g/mL and 0.8100 g/mL respectively. This suggested that azoxystrobin had as about 58 tunes of activity against P. cubensis as that of kresoxim-methyl.Population sensitivity of P. cubensis in greenhouse to azoxystrobin was monitored by the method described above, where the fungicide was successively used for eighteen to twenty four times in two growth seasons. The ECso of 47 isolates from the experimental greenhouse ranged from 0.00004 g/mL to 0.0110ng/mL, with a mean EC50 0.0022 g/mL. None any resistant isolate was detected. It implied the resistance risk of P. cubensis to azoxystrobin was not very high in the case of the fungicide successively application in two growth seasons.One metalaxyl high resistantce isolate Ml had an EC50 0.0025 g/mL to azoxystrobin. This result indicated that there was not positive cross resistancebetween azoxystrobin and metalaxyl, and azoxystrobin could be used for control the downy mildew caused by metalaxyl resistance P. cubensis.Results of field experiment showed that azoxystrobin was an excellent fungicide for control cucumber downy mildew. The effective concentration of dilution from 1667 to 1250 times of 25% azoxystrobin SC was recommended. Of the treatments, spraying the dilution of 1250 times provided more than 75% disease control efficiency. When it was applied at the early stage of downy mildew cycle and successively sprayed three times with an every seven days interval, the disease control efficacy could be kept idea.Azoxystrobin had strongly fungicidal and fungistatic activity to A. solani. Azoxystrobin inhibited mycelium growth, conidium sporalation and germination to this fungus, the EC50 of inhibition of mycelium and conidium germination was 37.23 g/mL and 0.0101 g/mL respectively, and conidium sporalation was inhibited 59.7% at the dose of 12.5 g/mL. 73 isolates totally collected from areas where strobilurin fungicide never been used were determined their sensitivity to azoxystrobin by utilizing a standard spore germination assay. Baseline sensitivity(EC5o) to azoxystrobin was 0.00507 n g/mL with ECso values ranged from 0.0007 u g/mL to 0.3232 u g/mL, and none any field resistance isolate was found.