Screening And Control Effect Of Biocontrol Bacteria Against Watermelon Fusarium Wilt Caused By Fusarium Oxysporum F.sp.niveum

Watermelon is one of the most important economic crops in the world,it is vulnerable to various pathogen in the growth and development stage,which affects the economic output.Watermelon Fusarium wilt caused by F.oxysporum f.sp.niveum is the most important soil-borne disease that damages watermelon yield.To achieve environment-friendly production and respond to sustainable development strategies,the methods of controlling watermelon Fusarium wilt,such as pesticides,resistance breeding and grafting should be transformed into the microbial agent.This experiment was conducted to isolate F.oxysporum f.sp.niveum,and screen the antagonistic bacteria for the watermelon Fusarium wilt,and analyze the control mechanism of antagonistic bacteria.The main contents as follows:F.oxysporum f.sp.niveum was isolated from the roots of diseased watermelon plants,the plate antagonism test was used to screen the antagonistic bacteria strain MM.By the mycelial growth rate method,the strain MM has a significant inhibitory effect on F.oxysporum f.sp.niveum,the inhibition rate was 65.46%,the hyphae appeared deformities such as swelling,constriction and fracture.The confrontation between strain MM and 9 pathogenic fungi showed that strain MM had broad-spectrum antibacterial activity.Based on molecular biology and combined morphological identification,the strain MM was identified as Serratia plymuthica.It was found that strain MM had various hydrolase activities by detecting the transparent circle,and the fermentation filtrate of the strain MM had an inhibition rate of 89.68%on the spore germination of F.oxysporum f.sp.niveum.The fermentation medium and culture conditions of strain MM were optimized,the optimal medium and conditions were:glucose 12.748 g/L,beef extract 2.086 g/L,yeast powder 3.091 g/L,p H7.0,temperature 30℃,rotation speed 180 r/min,inoculum size 3%,fermentation time 72 h.The EC₅₀ of S.plymuthica MM against F.oxysporum f.sp.niveum was 216.7μl/m L by measuring the activity of the fermentation filtrate.In the quantitative test of mycelial inhibition,the inhibition rate of MM against F.oxysporum f.sp.niveum reached 70.6%.The plate pair button method was carried out to detect the volatile substances of S.plymuthica MM,the results showed that has efficient antibacterial effect,the inhibition rate reached 71.66%.Besides,the metabolites of strain MM not only had high temperature resistance activity,but also against various soil-borne diseases,the inhibition rate ranged from 40.04%to 76.78%.The strain MM with GFP tag was constructed by genetic engineering technology,laser confocal microscopy results showed that strain MM could colonize watermelon roots.S.plymuthica MM can synthesize IAA,promote the growth of seeds and plants,and the yield was 10.239 mg/L.The seeds germination test suggested that S.plymuthica MM fermentation filtrate could promote seed germination,and the filtrate was diluted 150 times,which had the most significant effect on seed germination.The pot experiment proved that strain MM could effective control the watermelon Fusarium wilt,the control effect was 74.55%.The S.plymuthica MM can increase the total chlorophyll content and root vitality of watermelon plants by 1.36 times and 57.06%,respectively.The results showed that the expression of POD,PAL,C4H,LOX and CLPR3 genes were up-regulation to varying degrees,by the measurement of defense enzyme activity and the expression of related disease resistance genes after the treatment of biocontrol bacteria,indicating that S.plymuthica MM can activate the expression of multi-pathway resistance genes in the roots of watermelon plants,thereby enhancing the resistance of watermelons to F.oxysporum f.sp.niveum.The above research result showed that S.plymuthica MM had significant effect on promoting the growth and development of watermelon plants,and could effective control the watermelon Fusarium wilt,which had favorable biocontrol potential and development prospect.