Isolation And Screening Of Biocontrol Bacteria Against Southern Root-Knot Nematode And The Study Of Their Application Potential

Meloidogyne incongnita is an important member of Tylenchida,Heteroderidae,Meloidogyne,and it is a sedentary endoparasitic plant pathogenic nematode.The second instar larvae in soil invade from the host root and establish giant cells near the vascular bundle as feeding sites.The enlarged giant cells lead to root swelling and root knot,which leads to crop root-knot nematode disease.Meloidogyne incongnita can infect vegetables,beans,grains,weeds,shrubs and fruit trees and its host range is extremely wide.Chemical control is still the main method for root-knot nematode diseases,but chemical control often has some shortcomings,such as high toxicity and serious pollution of the ecological environment,and because of the singularity of chemical control agents,nematodes also exhibi drug resistance.Biological control has the advantages of environmental protection,pollution-free and sustainability and has gradually become the first choice in crop disease control.There are some shortcomings in the biological control of root-knot nematodes,such as less available biological control resources and poor field control effects.To enrich biocontrol resources,284 strains were isolated from vegetable soil in Yangzhou,and three strains with high nematicidal activity were screened out:HJ03,HJ04 and HJ07.The control effect,biocontrol mechanism and application prospects were studied.The main results were as follows:The corrected mortality rate of HJ03,HJ04 and HJ07 to the second instar larvae of Meloidogyne incongnita reached the highest after 24 h of treatment,and the fermentation broth diluted twice still had high nematicidal activity,which was 77.72±1.99%,78.56±2.40%and 88.05±3.63%,respectively.The results showed that the metabolites produced by the three biocontrol strains had nematicidal activity.The fermentation broth also inhibited the hatching of Meloidogyne incongnita eggs,and the relative inhibition rates were 71.14±0.67%,65.93±1.43%and 65.93±1.43%,respectively.Chemotaxis tests showed that fermentation broth treatment could significantly reduce the number of nematodes around the root tip of tomato.The number of nematodes near 1 cm of the root tip of the control treatment was 21.67,while that of the HJ03,HJ04 and HJ07 treatments was 2,2.67 and 1.33,respectively,indicating that the ability of nematodes to find hosts was disturbed.Further study showed that the number of nematodes,root knots and oocysts in tomato root tissue decreased significantly after irrigation with fermentation broth.The relative control effects were 60.04%,40.02%and 70.01%,and the oocyst decline rates were 75.73±7.97%,73.08±12.23%and 78.26±6.27%,respectively.After root irrigation with fermentation broth,the number of giant cells decreased,and the length and width decreased,which indicated that fermentation broth might inhibit the formation of root knots by inhibiting the establishment of nematode feeding sites.In addition,all three strains can alleviate tomato Fusarium wilt caused by Meloidogyne incongnita and Fusarium oxysporum,which may be caused by inhibiting the invasion of Meloidogyne incongnita.The germination index of seeds treated with HJ03,HJ04 and HJ07 fermentation broth was better than that of seeds treated with LB,which indicated that the fermentation broth of HJ03,HJ04 and HJ07 could promote seed germination.The plant height and root length increased significantly after 40 days of root irrigation,which indicated that the strain fermentation broth could promote the growth of plants.HJ03,HJ04 and HJ07 fermentation broth after 37℃-100℃,10 generations,24 hours ultraviolet irradiation,21 days storage,nematicidal activity did not change significantly,with good resistance to stress,genetic and storage stability,indicating that there is good potential for biocontrol agents.According to the results of 16S rRNA sequencing and sequence alignment,it is speculated that HJ03 is Bacillus amylooliquefaciens,HJ04 is Bacillus sapphorus and HJ07 is Stenotrophomonas maltophilia.The results indicated that HJ03,HJ04 and HJ07 were effective in inhibiting egg hatching of Meloidogyne incongnita,host search and invasion of second instar larvae,formation of root knot and egg mass,and complex disease symptoms of Fusarium.In addition,HJ03,HJ04 and HJ07 had the ability to promote plant growth and resist adverse environmental conditions such as high temperature and ultraviolet rays and had good application potential,which could be developed into biocontrol agents for crop root-knot nematode disease control.