Screening Of Biocontrol Strains Against Citrus Canker And Study On Reduction Of Pesticide Dosag

Citrus canker is a worldwide disease.After the plant is damaged,leaves,fruit drop,and tree vigor is weakened.Agriculture is mainly based on cultivation control and chemical control.However,the long-term application of bactericides has caused very serious damage to the environment,and even endangered human health.It is urgent to solve the problem of chemical pollution.In this thesis,the combination of biological control and chemical control is used to control citrus canker.While effectively preventing and controlling citrus canker,the dosage of bactericides is reduced to ensure the quality and safety of citrus products and reduce the pollution of bactericides to the environment.Pathogens are isolated from citrus canker leaves and identified as Xanthomonas citri subsp.citri,strain A.The turbidity method is used to determine the virulence of bactericides to pathogens,and the spectrophotometric method is used to determine the growth changes of the pathogens after the interaction between the pathogens and bactericides.The results show,20%Thiodiazole copper SA,20%Resin acid copper salt SA,78%Boer manganese zinc WP,12%Zhongshengmycin WP,80%Ethylicin EC,0.5%Berberine AS EC₅₀value are26.60μg/m L,18.80μg/m L,3.39μg/m L,3.01μg/m L,0.32μg/m L,0.0041μg/m L.The best application method is to give priority to Berberine,apply Ethylicin after 48 h,and reapply Berberine after 120 h.This application method is more conducive to the reduction of bactericides.The biocontrol bacteria FJ-19 which can inhibit the growth of citrus canker is successfully screened.Through morphological observation,physiological and biochemical characteristics analysis,16S r DNA sequencing comparison and phylogenetic tree construction,FJ-19 is identified as Sphingomonas.The combined antibacterial experiment show,pre-cultivating the biocontrol bacteria for 36 h,and the antibacterial effect of the biocontrol bacteria and Berberine are far better than that of Berberine and Ethylicin.Combined with the results of biofilm determination and glycoside hydrolase PCR identification,it is speculated that theα-glucosidase and glycoside hydrolase 15 family proteins in the biocontrol bacteria inhibit the growth of citrus canker by destroying or inhibiting the biofilm formation.At the same time,the fermentation conditions for protein production of biocontrol bacteria are optimized.The optimal conditions for fermentation of biocontrol strains are determined as and speed of 200 r/min,fermentation for 60 h,temperature 28℃,p H=6,inoculum size 6%,glucose 0.6%,tryptone 1.5%,magnesium sulfate 0.7%,and the highest protein yield is 2.022 mg/m L.Biocontrol agents are prepared with sodium alginate and calcium chloride as carriers,and the inhibition rate of biocontrol agents against citrus canker within 96 h is up to about89.2%.Taking the sensitive crop soybean as the research object,the seed germination rate,plant growth,chlorophyll changes,and rhizosphere microbial biomass and activity of soybean plants are measured before and after the application of biocontrol agents.The results show,the biocontrol bacteria can significantly increase the microbial biomass and activity in the rhizosphere of plants,and have a certain promotion effect on plant growth and development.In this thesis,biocontrol bacteria FJ-19 is successfully screened,and a more scientific and reasonable method of bactericides is proposed.the biocontrol mechanism of biocontrol bacteria is initially explored,and the influence of biocontrol strains on environmental safety is examined.These lay the foundation for the practical application of biocontrol bacteria to field control in the future.