Preliminary Study On The Microecology Of Biocontrol Agents To Alleviate Fusarium Wilt Of Watermelon Under Continuous Cropping

Watermelon in Ningxia is a unique selenium rich watermelon grown in the semi-arid region of northwest China,and is one of the main economic crops.However,continuous monoculture has led to a significant increase in Fusarium wilt caused by Fusarium oxysporum,and there is still a lack of effective control strategies.By collecting rhizospheric soil from different watermelon continuous cropping systems and using high throughput sequencing technology,the microecological structure（bacteria and fungi）was analyzed to determine the population dynamics of Fusarium in soil at different monocropping stages;On this basis,through plate isolation and gene sequencing,typical strains of Fusarium wilt pathogen were isolated from plant residues in the main watermelon production area of Ningxia,and their nutritional and pathogenic characteristics were clarified;Using the biocontrol microorganisms Bacillus amyloliquefaciens B6,Pseudomonas brassicacearum WF19,and their composite microbial combination BW selected by the research team in the early stage,the pot experiment was conducted to verify their disease prevention effect against Fusarium wilt,and to explore the effects of biocontrol microorganisms on nitrogen,sugar metabolism,antioxidant enzymes,and lipid peroxidation in watermelon plants;Based on the results of pot experiments,field experiments were conducted to determine the biocontrol effects of Bacillus amyloliquefaciens B6,Pseudomonas brassicacearum WF19,and their combined microbial combination BW in the field,and to explore their microecological regulatory mechanisms for the occurrence of watermelon wilt.The main research results are as follows:（1）The activities of phosphatase,urease,and sucrase in long-term continuous cropping soil decreased significantly;watermelon cultivation increased soil alkalinity,but long-term continuous cropping did not cause a further increase in soil alkalinity;during continuous cropping,bacterial diversity increased,but fungal diversity decreased;Dispersal Limit（79.87%-100%）mainly affects the structural variation of the overall bacterial community and different continuous cropping sub communities;in short-time continuous cropping soils,Dispersal Limit（40%-90%）and Heterogeneous Selection（10%-30%）mainly affected the structural variation of the overall fungal community and different continuous cropping sub communities;compared with the OTU of uncultivated soil XSG0,the average OTU of bacteria increased by 12.4%and decreased by 28.9%;the situation in the fungal community is just the opposite,with the average upward and downward response OTU of 24.2%and 13.6%,respectively;after continuous monoculture for 23 years,the relative abundance of Fusarium decreased from 4.41%in the uncultivated control XSG0 soil to 0.61%in the XSG23 soil.NO₃^--N（-0.87）and AP（-0.57）were identified as the most important determinants of Fusarium,and Bacillus sp.was one of the bacterial species enriched in continuous cropping.（2）BMD-1 was Fusarium oxysporum,which grows best in sabouraudmedium and potato dextrose agar media,with growth rates of 0.8604 cm/d and 0.8150 cm/d,respectively that the mycelium was deep purple and dense;Glucose was the best carbon source;Yeast powder and peptone significantly promoted the growth of BMD-1 at an optimal temperature of 28℃;BMD-1 had a higher tolerance to alkaline environments than acidic environments,with an optimal pH of 8.0 and growth at pH of 12;In addition,nitrogen and potassium significantly increased the pathogenicity of BMD-1,while magnesium had a certain increase in the pathogenicity of BMD-1;Phosphorus and calcium could significantly reduce the pathogenicity of BMD-1.（3）Bacillus amyloliquefaciens B6,Pseudomonas brassicacearum WF19,and their combined microbial combination BW increased the contents of NO₃^--N and NO,and the activities of NR,GDH,and GS in the root and leaf tissues of watermelon plants;activities of sucrose synthase S-I and S-II;activities of POD,PPO,and PAL and lignin content decreased content of MDA and NH₄⁺-N in root and leaf tissues of plants.Biocontrol strains regulate the resistance of watermelon plants to BMD-1 through regulating carbon,nitrogen metabolism,antioxidant enzymes,and lipid peroxidation,ultimately improving the ability of watermelon plants to resist Fusarium wilt.（4）Biocontrol agent B6 increased the activities of four biological enzymes and the contents of TN,TP,AP,NO₃^--N,NH₄⁺-N and OM in soil,while decreased the contents of AK and TK;WF19 increased the activities of CAT,ALP,and SUC in soil,decreased the activities of URE and TN,and increased the contents of TP,AP,TK,AK,NO₃^--N,NH₄⁺-N and OM in soil;BW increased soil content of NO₃^--N,activities of CAT and SUC,decreased URE and ALP activities,and decreased soil content of TN,TP,TK,AP,AK,NO₃^--N and OM.After applying biocontrol agents to the roots,the relative abundance of Fusarium in the soil treated with WF19and BW decreased,while the relative abundance of Gibberella in the soil treated with B6decreased;the content of AP,TP,and activity of URE have the greatest correlation with bacterial community structure,while the content of AP,TK,and URE activity have the greatest correlation with fungal community structure.Nitrate respiration,nitrogen fixation,and nitrite ammonification have negative effects on Fusarium oxysporum,with effect coefficients of-2.27,-1.55,and-0.57,respectively.In summary,long-term continuous cropping can change the soil microecological structure and enrich some pathogenic bacteria of plant diseases;the resistance metabolism of watermelon plants,environmental factors in rhizospheric soil,and the structure and function of rhizospheric soil microecology jointly mediate the regulatory process of biocontrol strains inhibiting watermelon wilt.