Screening And Fermentation Of Antagonistic Bacteria Against Fusarium Wilt Of Cucumber And Its Biocontrol Mechanism Studying

Cucumber is an economically important and popular vegetable crop.As the continuous cropping area of cucumber is increasing,soil-borne diseases,especially Fusarium wilt,are increasing year by year.Fusarium wilt caused by Fusarium oxysporum f.sp.cucumerinum J.H.Owen?FOC?is one of the most destructive soil-borne diseases causing large economic losses worldwide,affecting the quantity and the quality of cucumber.Chemical control using fungicides has been the most common control strategy for managing diseases caused by Fusarium spp.However,their extensive usage have not only caused environmental and toxicological problems but also jeopardized human and animal health.Furthermore,the application of chemical fungicides is known to select for drug resistant strains,limiting their long-term effectiveness.Therefore,an efficient and safe method to prevent and control Fusarium wilt disease is urgently needed.Biological control is an eco-friendly,sustainable,and safe method for controlling plant infections,and has become one of the reliable alternatives to chemical fungicides.1.A total of 76 bacterial isolates were isolated from the rhizosphere soil.Of them,8 bacterial isolates showed antagonistic effect on F.oxy.sporum.Strain HX-140 had the best inhibitory effect on F.oxysporum and the inhibition rate of strain HX-140 on F.oxysporum was 76.53%.Based on the morphological,physiological,and molecular characterizations,strain HX-140 was identified as Paenibacillus poly,myxa.P.polymyxa HX-140 displayed a broad spectrum of antifungal activity against Fusarium oxysporum,Sclerotinia sclerotiorum,Colletotrichum lini,Fusarium lini,Phytophthora capsici,Colletotrichum gloeosporioides,Rhizoctonia solani,and the inhibition rate of P.polymyxa HX-140 on the 6 pathogens was more than 60%.Strain HX-140 does not produce IAA and ammonia,and can't dissolve phosphate.Strain HX-140 can grow in the environment like 10?-40?,pH 2-pH 10 and containing 0%-5%NaCl.2.A preliminary study on the antagonistic mechanism indicated that the strain HX-140 had extracellular protease,cellulase and ?-1,3-glucanase activities,was able to produce anti-fungal volatiles,but absence of chitinase activity and anti-fungal activity in the sterile fermentation broth.3.The effects of strain HX-140 and Fusarium wilt of cucumber on cucumber defense response enzymes?CAT,POD,PPO,PAL,SOD?were studied.The results showed that the activities of CAT,POD,PPO,PAL and SOD in cucumber leaves could be increased within 48 h-72 h by applying either culture solution of F.oxysporum or strain HX-140 suspension alone,or applying culture solution of F.ox)ysporum and HX-140 suspension simultaneously.The peak value of applying culture solution of F.oxysporum and HX-140 suspension simultaneously was higher than the peak of applying culture solution of F.oxysporum or strain HX-140 suspension alone,and the activities of CAT,POD,PPO,PAL and SOD in cucumber leaves were 6.14,1.57,1.87,1.88 and 2.73 times higher than those in untreated cucumber leaves.In addition,the activities of defense enzymes in cucumber leaves increased again in varying degrees after 3-4 days.4.The optimal medium composition and culture conditions of strain HX-140 producing spores were obtained by response surface optimization:tryptone 19 g·L^-1,bran 25 g·L^-1,Na₂HPO₄ 20 g L^-1,pH 7,inoculation concentration 3%,liquid volume 100 mL,180 r min-1,27? for 48 h.Under this culture condition,the spore concentration of strain HX-140 was 2.69×107 cfu·mL^-1,which was 16.3 times that before optimization.5.The pot experiment of strain HX-140 showed that the control efficiency of strain HX-140 against Fusarium wilt of cucumber was 55.56%,which laid a foundation for field experiments.6.The field experiment of strain HX-140 showed that the efficacy of the different concentrations(10⁶,10⁷ and 10⁸ cfu·mL^-1)of bacterial fermentation broth was evaluated and the disease control efficacy was 19.54%,41.10%,and 50.85%,respectively.Strain HX-140 also had a greater impact on the number of different microorganisms that can be cultured in the soil,increasing the number of bacteria and actinomycetes in the soil,reducing the number of fungi,which laid a foundation for further application of strain HX-140.