Identification and biocontrol effect of two Bacillus strains against root-knot disease
Root-knot disease,caused by Meloidogyne incognita,seriously threatens the production of many crop plants.To further explore biocontrol resources for this disease,two bacterial strains showing nematicidal ac-tivity were isolated from vegetable rhizopsheric soil in Yangzhou City,China.These two strains,named HJ03 and HJ04,were identified as Bacillus amyloliquefaciens and B.safensis,respectively,based on their morphologi-cal characteristics,biochemical properties,and phylogenetic analysis results.The corrected mortalities of HJ03 and HJ04 cell suspensions against second-stage juveniles(J2s)of M.incognita after 24 h of treatment were 77.3%and 79.3%,respectively,while those for culture filtrates of the two strains were 75.9%and 80.1%.No significant difference in corrected mortalities was observed between treatments with cell suspension and culture filtrate.Cell suspensions of HJ03 and HJ04 cultured for 24 h showed good nematicidal activity,and extended culture time did not significantly increase nematicidal activity.Cell suspensions(OD600=1.2)of HJ03 and HJ04 diluted to 1:2 resulted in mortalities of 77.7%and 78.6%,respectively,similar to those caused by their mother solutions,but showed significantly decreased mortalities when diluted to 1:5.The biocontrol mechanisms of HJ03 and HJ04 against M.incognita were revealed as follows:HJ03 and HJ04 can inhibit egg hatching of M.in-cognita,with inhibition rates of 71.1%and 64.0%,respectively,compared to LB medium-treated control;sig-nificantly inhibit the chemotaxis of J2s.The number of J2s in tomato rhizosphere(1 cm)treated with HJ03 and HJ04 was reduced by 90.9%and 87.6%,respectively,compared with that of control;dramatically inhibit nema-tode infection,with inhibition rates of 85.4%and 85.2%,respectively,after treated with HJ03 and HJ04 for one week;inhibit gall formation,with gall indexes of 1.1 and 1.6 for HJ03 and HJ04,respectively,at 40 d after treatment,whereas 3.3 for control;inhibit the formation of egg masses,with inhibition rates of 75.7%and 73.1%for HJ03 and HJ04,respectively,at 60 d after treatment;reduce the size of M.incognita-induced giant cells.In addition,HJ03 and HJ04 can promote seed germination and seedling growth of tomato.This study enri-ches the biocontrol resources against root-knot disease and lays a basis for further development of biocontrol agents.