Formulation Of Bacillus Velezensis F85 And Its Evaluation Of Biocontrol Efficacy On Plant Disease

Bacillus spp.is an important biocontrol microbial resource in agricultural production and has antagonistic effects on a variety of plant pathogens.In recent years,microbial fungicides and fertilizers made from Bacillus spp.have played an important role in plant disease control.In this study,we optimized the fermentation conditions for the industrial production of F85（Bacillus velezensis）,prepared F85 wettable powder,evaluated the biocontrol efficacy of the fermentation solution and wettable powder of F85 strain on the rapeseed clubroot and rice blast,and further investigated the biocontrol mechanism of F85strain on rice blast.The main results are described as follows:1.The optimal fermentation process of B.velezensis F85 was clarified.The results of the effects of different media,loading volume,inoculum volume,rotational speed,p H,temperature and fermentation time on the spore number of F85 strain showed that the optimal fermentation media were 24 g/L corn flour,6 g/L yeast powder,15 g/L yeast paste,5 g/L corn slurry and 5 g/L sodium chloride and the optimal fermentation conditions were6.5%inoculum volume,40%loading volume,rotational speed 230 rpm,37℃,p H8.0 and48h fermentation.The most suitable carrier for the wettable powder was kaolin,and the fermentation solution was mixed with the carrier kaolin in a suitable ratio and spray dried to obtain F85 wettable powder with spore concentration of 5×10¹⁰ cfu/g.2.The control efficacy of B.velezensis F85 on rapeseed clubroot was evaluated.The results of indoor pot trials showed that the biocontrol efficacy of 5×10¹⁰ cfu/g F85 wettable powder seed dressing treatment,20%F85 fermentation solution seed dressing and 10%cyanoxazole seed dressing reached to 67.06%,31.76%and 56.47%on rapeseed clubroot.In the field trials of Jingzhou city in 2022,the biocontrol efficacy of 5×10¹⁰ cfu/g F85wettable powder seed dressing and 1×10⁹ F85 cfu/g F85 soil granules reached to 40.66%and 49.84%,and the biocontrol efficacy of combined treatment of F85 wettable powder seed dressing and F85 soil granules reached to 68.52%,which was not significantly different from the biocontrol efficacy of combined treatment of 10%cyanoxazole seed dressing and root irrigation.3.The control efficacy and mechanism of B.velezensis F85 on rice blast were investigated.The results of indoor plate test showed that the fermentation filtrate of F85 at eight concentrations had significant inhibitory effects on the mycelial growth of Magnaporthe oryzae.After treatment with F85 fermentation filtrate,the inhibition rate of mycelial growth of M.oryzae was 17.7%-45.5%,and the inhibition rate of mycelial dry weight of M.oryzae was 27.06%-75.56%.The treatment of F85 fermentation filtrate could effectively inhibit the conidial germination,appressorium formation and the expansion of infection hyphae.The results of experiment in pot showed that the biocontrol efficacy of three concentrations of F85 fermentation solution and three dilutions of F85 wettable powder on rice blast at three application time points of the same time and before and after inoculation of M.oryzae 24 h were 23.73%-58.76%and 33.90%-62.15%,respectively.The control efficacy of 1:250 dilution of F85 wettable powder at three application time points was 56.50%-62.15%,which was not significantly different from that of 75%tricyclazole.The results of dynamic detection of rice defense-related enzyme activity and defense-related gene expression showed that the activities of defense enzymes PPO,POD and SOD in rice leaves increased to varying degrees after F85 wettable powder treatment for 1 d,2 d,3 d,5 d and 7 d.Compared with the control,the activities of PPO,POD and SOD increased by 2.68 times,2.5 times and 4.41 times respectively when reached the peak in the simultaneous treatment of F85 wettable powder and M.oryzae.The expression of PR1a,PR1b,PR3,PR4,PR5,PR10 and NH1 genes in rice leaves were up-regulated at different time points.Therefore,the treatment of rice leaves with F85 strain induced the increase of related defense enzyme activities and the up-regulated expression of related defense genes,thereby improving the resistance of rice to rice blast.