The Studies On The Identification, The Induced Resistance Against Cucurbits Powdery Mildew And The Biological Safety Of Biocontrol Agent LJ1

Powdery mildew is one of the serious disease on cucurbits over the whole world,which causees heavy loss and reduce the quality of melon and cucumber. It mainlydamages leaves, also can infect the stem and petiole. Cucurbits powdery mildewusually occurs in the growth later period and results in a decline in the fruit qualityand yield loss. The biocontrol agents could reduce the application of agrochemicalsfor controlling plant disease and delay the development of fungicide resistance inpathogen population. The methods of biological control have been widely recognizedas new safe and efficient means of disease control.Biological control agent LJ1is a bacterium isolated from soil samples andexhibits good effects against melon powdery mildew. The results of field trialsshowed that LJ1is powerful in controlling cucumber powdery mildew. The14-daycontrol efficacy of LJ1against powdery mildew in cucumber was83.45%. This papermainly aimed at clarifying the classification statue, induced resistance, control effects,biocontrol mechanism and the influence on animals and plant of strain LJ1.Based on morphological characteristics and phylogenetic analysis of16S rDNAsequences, strain LJ1was primarily identified as the genus of Bacillus. Then, bytesting anaerobic, growth temperature, salt resistance, antibiotics sensitivity, motility,produce acid from carbohydrate and alcohols, utilization of carbon source and othercharacters, and referring to the ‘Bergey’s Manual of Systematic Bacteriology’, thestrain was determined as Bacillus amyloliquefaciens.In order to explore the biocontrol mechanism of LJ1, cucumber seedlings weresprayed with100-fold supernatant of LJ1fermentation broth and then the activitytrend of superoxide dismutase(SOD), peroxidase(POD), polyphenol oxidase(PPO),phenylalanine ammonia lyase(PAL) and salicylic acid(SA) in cucumber leaves weredetermined. The results showed that all resistance index have a jump process at certain time point and the activity increased significantly comparing with that of thecontrol group after being treated with supernatant of LJ1, and the content of SA incucumber leaves kept growth from3d, with significant difference (P<0.01) comparingwith the control group. The results of present study suggested that some componentsin the supernatant of LJ1fermentation broth could induce systemic resistance incucumber seedlings.To explore the safety to mice, fermentation broth of LJ1was conducted withacute toxicity test and observing whether there is lethal effect on mice, then detectingaspartate aminotransferase(AST), alanine aminotransferase(ALT), alkalinephosphatase(ALP) activity and6hemogram indexes. The results showed thatbiocontrol agent LJ1shares low toxicity on mice in both stomach and skin toxicitytests(LD50>4640mg/kg); AST, ALT, and ALP activity in serum of mice increasedwith significant difference (P<0.05) comparing with the control group after3days.All of the indexes recovered and there were no significant difference with controlgroup (P<0.05) after14days. It was proved that the fermentation broth of LJ1hasside effect on the viscera function of mice at preliminary stage, but the metabolismwould restore to normal14d later. So it could be deduced that the biocontrol agentLJ1is safe to animals.The cucumber seeds were soaked and the seedlings were sprayed by differentconcentration of LJ1fermentation broth, then observing the growth of cucumberseedlings and detecting the microbial community dynamics of the rhizosphere andleaves. The results showed that biocontrol agent LJ1had no effect on cucumbergermination. After seed soaking and spraying on the leaves by fermentation broth, thecucumber seedlings growth conditions were significantly better than control. The100-fold fermentation broth had the best growth-promoting effect on the height, stemcircumference, fresh weight and dry weight, and these indexes were significantdifference with control group (P<0.01).The bacteria biomass of rhizosphere andleaves decreased with the increase of dilution ratio, and most of the bacteria weresimilar with biocontrol LJ1. It can be inferred that LJ1may have colonization ability;spraying on the leaves had less influence on the actinomycetes, but the number ofactinomycetes increased significantly after soaking by low concentration of LJ1fermentation broth; all of the treatment showed good effect against fungi on therhizosphere and leaves. It suggested that the fermentation broth of LJ1could inducesystemic resistance in cucumber seedlings and the induction-secreted activesubstances in cucumber rhizosphere is broad spectrum against fungi.