Screening Of Antagonists Against Soil-Borne Root Rot Of Sanqi Ginseng And Its Biological Control Influences

Sanqi ginseng（Panax notoginseng（Burkill）F.H.Chen）is a precious perennial Chinese herbal plant of Araliaceae,which is mainly cultivated in some areas of Yunnan and Guangxi province.Sanqi root rot is a complex soil-borne disease,as the pathogens include fungi,bacteria,nematodes and other microorganisms.Fusarium oxysporum is a typical pathogen that causes plant root rot.Most of the biocontrol control methods try to isolate beneficial fungi and bacteria which are antagonistic against plant pathogens,so as to inhibit the pathogen population densities in the plant rhizosphere.Therefore,in this study,two strains named KW6 and KW8 with strong antagonistic ability to Fusarium oxysporum（FO）,the main pathogen of root rot disease,were isolated from the rhizosphere soil of sanqi.The optimal culture and nutrition conditions of the two antagonistic bacteria were explored.Different concentrations of antagonistic bacteria and a certain concentration of FO were added to the soil in pot experiment.The effects of strains KW6 and KW8 on biocontrol effects of sanqi root rot mainly caused by Fusarium oxysporum,and their influnces on plant growth and rhizosphere microflora were determined.In addition,the effects of toxins produced by pathogen FO on the growth,development and main antagonistic ability of the two antagonistic bacteria were also further determined.The results are shown as follows:1.Two antagonistic bacteria were isolated from the diseased sanqi rhizosphere,which showed strong inhibitory effect on the growth of pathogen FO.After molecular identification,strain KW6 was identified as Bacillus velezensis and KW8 as Lysobacter enzymogenes.The antagonistic strain KW6 exhibited the best growth condition when the liquid volume was 50m L/250m L,and it was more acid-tolerant and less alkali-tolerant.The most suitable carbon source was fructose and the most suitable nitrogen source was beef paste.The antagonistic strain KW8 exhibited the best growth condition when the liquid volume was 75m L/250m L,and the growth and development was normal when p H was 5-8.The most suitable carbon source was sucrose and the most suitable nitrogen source was yeast powder.2.In the pot experiment,it was found that the antagonistic bacterial strains KW6and KW8 showed no pathogenicity to the sanqi roots.Both 10~6 and 10~8 concentrations inoculation could effectively control the incidences of sanqi root rot and promote the growth of different parts of sanqi itsself,and could also increase the fresh weight of sanqi,especially the root.The growth and development promotion increased with the increase of the inoculated concentration of antagonistic bacteria.Application of 10~8concentration of antagonistic strains KW6 could reduce the number of fungal OTU in sanqi rhizosphere soil,and reduce the number and species richness of fungi,but not significantly change the fungal community structure.The amounts of bacterial OTU in sanqi rhizosphere soil were also decreased by antagonistic bacteria KW6 and KW8added at certain concentration.The number,richness and diversity of bacterial species were decreased by using B.velezensis KW6.Compared with Control group,different concentrations of antagonistic strains KW6 and KW8 significantly reduced the relative abundance of potential pathogenic Alternaria.Compared with the Control group,the application of different concentrations of antagonistic bacteria did not significantly change the functional units of the fungal flora in the sanqi rhizosphere soil but there were significant differences among different concentration groups.In addition,10~8 of B.velezensis KW6 and L.enzymogenes KW8 inoculation could significantly change the functional units of the bacterial flora.The results of metabolic pathways showed that the metabolism of fungal flora in the sanqi rhizosphere soil were mainly concentrated in biosynthesis function and generation of precursor metabolite and energy function,while the metabolic pathway of bacterial flora was mainly concentrated in biosynthesis function.The results of FO colonization in sanqi plants showed that antagonistic bacteria could significantly reduce the number of pathogens in stems and roots,L.enzymogenes showed a more obvious control effect on the number of pathogens,and the number of FO colonization decreased with the increase of antagonistic bacteria concentration.3.In this study,the genomes of antagonistic bacteria B.velezensis KW6 and L.enzymogenes KW8 were sequenced.The results showed that the total sequence number and sequence length of B.velezensis KW6 were lower than that of L.enzymogenes KW8.Among the two functional enzymes of antagonistic strains,there were more genes for glycosyltransferase（GTs）,carbohydrate lipase（CEs）and glycoside hydrolase（GHs）.Both B.velezensis KW6 and L.enzymogenes KW8 can produce different amounts of cellulase,β-1,3-glucanase and chitinase,which can significantly reduce the population density of FO at the same level of enzyme activity.The co-culture of FO and angonistics bacteria only increased the cellulase production capacity of B.velezensi KW6,but showed no significant effect on the enzyme production capacity of L.enzymogenes.The production capacity of enzyme such as cellulase,β-1,3-glucanase and chitinase of strains KW6 and KW8 could be induced by inactivation of FO addition.Compared with the control group,the growth,enzyme production and biofilm formation of B.velezensis KW6 were significantly inhibited when the fusaric acid（FA）was 5μg/m L.When the fusaric acid was 5μg/m L,both the growth and enzyme production abilities of L.enzymogenes KW8 were significantly inhibited.When the FA was 1μg/m L,the biofilm formation ability of KW6 and KW8was significantly inhibited.