Screening,Identification,Optimization Of Culture Conditions And Biocontrol Mechanism Of Antagonistic Bacteria Against Powdery Mildew Of Momordica Momordica

Bitter melon has important economic value and medicinal prospects,and its production is seriously threatened by powdery mildew.In order to enrich the control measures of powdery mildew in bitter gourd and provide a new method for the biological control of powdery mildew in bitter gourd instead of chemical pesticides,the plate dilution method was used to isolate microorganisms in soil samples,and the in vitro leaf disc method was used to screen the strains with control effect against（Podosphaera xanthii）.The biological classification status of the strain was determined by morphology,biochemical characteristics and molecular biology,and the biocontrol effect of the strain was further determined by pot experiment.Finally,the control mechanism was explored,the results are as follows:（1）The microorganisms were isolated and purified from soil samples collected from Bawangling Nature Reserve,Limushan National Park,Jianfengling National Park and balsam pear plantation in Hainan Province,and an antagonizing strain of balsam pear powvermicella alba N46 was screened from 292 isolated strains by in vitro leaf disc method.It was identified as（Bacillus velezensis）by morphological characteristics,physiological and biochemical tests,and phylogenetic tree constructed by 16S r DNA.（2）The in vitro control effect of N46 fermentation broth isolate was tested,and the dominant position of bacterial suspension in the control effect was established,and the application concentration of 6×10⁸ CFU·m L^-1 was determined to achieve good effect.In pot experiment,the relative control effect of N46 suspension was 59.0%,which was similar to that of triazolone.The results showed that high concentration of antagonistic bacteria could inhibit the germination of bitter gourd seeds.The effect of high concentration of antagonistic bacteria on growth of bitter gourd at seedling stage was the best,the plant height,stem diameter and root index were significantly improved,and the biomass was increased by more than 27%than the control group.（3）The best carbon source was yeast extract and the best nitrogen source was beef extract in order to obtain the highest viable concentration.The optimal medium formula was predicted and tested by response surface method:beef extract 0.93%,yeast extract 0.52%,sodium chloride 0.50%.The optimal culture environment was found by single factor test:shaking speed 220 rmp,liquid volume 35%,inoculum volume 5%,initial p H 6,culture temperature 32℃,culture time 24 h.（4）The experimental results of exploring the prevention mechanism show that:The germination of foliar powdery mildew spores of bitter gourd treated with antagonist was 39%lower,and the activities of defense enzymes POD,CAT,SOD,reactive oxygen species（ROS）accumulation,allergic necrosis and lignin accumulation were significantly increased（p<0.05）compared with the control group in the face of powdery mildew infection.The antagonist-treated group showed higher expression of disease-resistance related genes（JA reactive marker genes,SA reactive marker genes,peroxidase genes）in a faster time and a stronger response when encountering pathogen infection.Finally,the production of disease resistance was inhibited by blocking JA route through IBU,suggesting that the improvement of disease resistance of bitter melon by antagonistic N46 may depend on JA pathway.In conclusion,the antagonist N46 has a good control effect on powderella alba,which is equivalent to the effect of commercial pesticides,and has an obvious growth promotion effect on bitter gourd.Antagonist N46 can control powdery mildew mainly by inhibiting the germination of powdery mildew spores and inducing resistance dependent on JA pathway,which has a good application prospect.