Risk Evaluation Of Resistance To Mandipropamid Of Phytophthora Capsici And Studies On Its Physiological And Biochemical Mechanism

Pepper Phytophthora blight is a devastating diseases, which can bring huge losses of pepper.So far, chemical control is still a main means of control of the disease in the production, but the long-term use of a single class of fungicides may result in occurrence of resistance to fungicides and low control efficacy. Carboxylic acid amide fungicide mandipropamid is a new type of Oomycetes fungicide and has not been used at large scale in China. Therefore, this study was undertaken on detection of sensitivity to metalaxyl and dimethomorph and cross-resistance relationship between metalaxyl or dimethomorph and the other fungicides with different mechanisms in Phytophthora capsici, the biological activity of mandipropamid against P. capsici, the sensitivity baseline to mandipropamid in P. capsici, risk assessment of resistance to mandipropamid and the mechanism of the physiological and biochemical mechanism of resistance to mandipropamid. The main results are as follows:1 Sixty-four isolates of P. capsici collected from different regions of Hebei province and Shijiazhuang were tested by measuring mycelial growth rate in order to explicit the sensitivity of P. capsici to metalaxyl and dimethomorph. Sensitivity of 11 relatively sensitive isolates and 3 relatively resistant isolates to metalaxyl, mandipropamid, dimethomorph, azoxystrobin, and cymoxanil were determined by the same method to make sure that it existed cross resistance or not between metalaxyl and other diverse active mechanism fungicides. The results showed that metalaxyl to P. capsici has obvious inhibition, EC₅₀ of 0.1661 27.5572μg /mL, the highest and lowest difference of 165.91 times. Collected from different regions strains of P. capsici sensitive to metalaxyl there were very different.The linear regression showed that there were no positively-correlated cross resistance between metalaxy and other diverse active mechanism fungicide. Dimethomorph on in the fully sensitive state.2 Sensitivity of 11 relatively sensitive isolates and 3 relatively resistant isolates to metalaxyl, mandipropamid, dimethomorph, azoxystrobin, and cymoxanil were determined by measuring mycelial growth rate;Protective effect ,curative of control of mandipropamid against pepper blight were separately determined by detached leaves test and potted plant spray test, and duration of control of mandipropamid against pepper blight were determined connection by fungus cake. The results showed that, mandipropamid and dimethomorph of the CAA fungicides have the best inhibition and the highest toxicity of P. capsici, followed by azoxystrobin of the strobilurin fungicides, cymoxanil has the worst. at the concentration of 10μg/mL, the preventive and curative action of mandipropamid against pepper blight were 86.67%,60.00% by detached leaves test, at the concentration of 100μg/mL, the preventive and curative action of mandipropamid against pepper blight were 95.25%,77.81% by potted plant spray test. With the 100μg/mL relative efficacy is still as high as 65.92% after 14days, and intervals up to 14days. General way to analyze, mandipropamid has good inhibition, but also has good control effect and duration. Its good effects also shows that mandipropamid has good absorption within the conduction.3 The sensitivity to mandipropamid of 173 P. capsici strains in Dingxing and Gaocheng of Hebei Province and Shouguang and Qingzhou of Shandong Province were determined by mycelia growth inhibition method. The results demonstrated that EC₅₀ value ranged from 0.0027 to 0.0499μg /mL, the difference between maximum and minimum are 18.48 times. With a mean value of (0.0219±0.0102)μg/mL. The sensitivity frequency of P. capsici to mandipropamid distributed as a single peak curve. The data could be used as baseline sensitivity of P. capsici to mandipropamid.4 The 10 strains were separately treated by UV- irradiating with colony point change and taming, with 7 and 1 resistant mutants to mandipropamid separately obtained in vitro by UV-irradiating and taming; The resistant levels ranged from 41.50 to 394437.77 times. Mutation frequencies were separately 1.46% and 1.43%. It was proved that mandipropamid has not cross resistance with dimethomorph, metalaxyl, azoxystrobin, and cymoxanil by comparing the sensitivity of different isolates to different fungicides and analysis their linear relation. 8 resistant mutants to mandipropamid were cultured in the CA medium without fungicide for 11 generations, the resistant trait were stably inheritable in 2 isolates, and their EC₅₀ were 1429.5237μg/mL and more than 5000μg/mL , but not stably inheritable in 6 isolates. Some biological characteristics of mutants were studied in laboratory. The mutants exhibited diversity in mycelial growth rate, pathogenicity, sporulation ability, fitness and competition ability with their parental strains. The sporulation, fitness and competition of DX 2-3 20m and DX 2-3 F7 were higher than the parental strains, contributing to the formation of resistance strains. Based on the above, it was suggested that P. capsici had a moderate resistance risk to mandipropamid . Therefore, we can conclud that resistant mutants can emerged quickly once mandipropamid was often used. It was suggested that the mandipropamid should be used alternately with other fungicides without cross-resistance to avoid or delay the development of resistance.5 Comparison resistance mechanism of P. capsici isolates with parental and resistant to the mandipropamid, determined on physiological and biochemical indexes were done.The results indicated that in the NaCl and glucose concentrations, isolates with parental and resistant for mycelial growth capacity was less affected, and at different concentrations, all strains show no significant osmotic pressure difference. It can be drawn NaCl and glucose on capsici does not have to provide nutritional and osmotic inhibition. Although mandipropamid in low concentrations resistant strains in vivo electrolyte to leak out more, with the treated time and concentration increase, the resistant strains inclusions less leakage. At the same time Strains with parental and resistant that soluble protein content andβ-1,3- glucanase activity change, parental strains of soluble protein content and cellβ-1,3- glucanase activity is higher than resistant strains. With the treated time increase, all strains of the soluble protein content andβ-1,3- glucanase activity tended to decrease. It can conclude that the inclusion of P. capsici is leakage through the cell membrane to accelerate metabolism, reduce the accumulation of mandipropamid, so that reach the actual target concentration was decreased. At the same time the body of the soluble protein content andβ-1,3-glucanase activity decreased. For these reasons may be resistant mechanism to mandipropamid.