Pathogen Identification Of Cucumber Bacterial Stem Soft Rot And Screening Of Resistant Lines

Cucumber bacterial stem soft rot disease has occurred in the main producing areas in China and caused serious economic loss.Cucumber bacterial stem soft rot occurs mainly in greenhouses and nursery sites,resulting in a reduction of 30% to 50% in production,and in severe cases leading to total failure.It has been reported that Pectobacterium carotovorum subsp.Brasiliense can cause cucumber bacterial stem soft rot in China.However,the pathogen responsible for cucumber bacterial stem soft rot in Jinan City,Shangdong Province,has not been identified and the incidence characteristics of cucumber bacterial stem soft rot has not been clear.In this study,the pathogen causing cucumber bacterial stem soft rot in Jinan City was determined by morphological observation,physiological and biochemical detection and sequence comparison of bacterial 16 S rDNA and resistant cucumber resources were identified.The main findings are as follows:Four representative strains(DT1,DT2,DT3,DT4)were isolated from the collected samples and their colony morphologies on LB solid medium were observed.DT1’s colony is yellow,convex,with smooth and wavy edge;DT2’s colony is light yellow,with smooth and glossy surface;DT3’s colony is white,convex,with smooth edge and no glossy surface;DT4’s colony is white,round,smaller and can produce yellow bacteria pus.The extracted genomic DNA from 4 isolates was used as a template for PCR amplification of 16 S rDNA using universal primers,respectively.The obtained sequences were compared in the NCBI database.The results showed that DT1 and DT2 shared 99-100% of sequence homology with many members in Aeromonas at nucleotide level;DT3 had a 100% of identity to Pseudomonas fluorescens at nucleotide level;DT4 had a 99% of identity to Enterobacter cloacae at nucleotide level.DT1,DT2,DT3,and DT4 were injected into the cucumber cotyledon.The observation for symptoms indicated that no obvious symptoms were found in the DT1,DT2 and DT4-inoculated leaves.5 days post inoculation in the cotyledon,many lesions appeared in the DT3-inoculated main leaves and the incidence was 30%;the incidence was up to 50-60% and stems were soften 7 days post inoculation while the incidence was 90% and the plants were flatten10 days post inoculation,suggesting that DT3 can cause cucumber stem soft rot.In addition,DT3 was isolated from the infected leaves according to Koch’s rule.In conclusion,the above-mentioned results verified that the pathogen causing cucumber bacterial stem soft rot in Jinan City is P.fluorescens.Using DT3 to inoculate leaves from 35 cucumber lines,the incidence and disease index were investigated.The results demonstrated that different lines displayed different resistance to bacterial stem soft rot.Among them,2 lines were highly susceptible,25 lines were medium susceptible and 10 lines were medium resistant.