The Suppressiveness Caused By Long-term Continuous Cropping Of Soybean On The Root Rot And Pathogens

As the primary root disease in the continuous cropping field of soybean, soybean root rotis one of the principal factors that causing the continuous cropping obstacles in NortheastChina. However, the situation of soybean root rot disease is amended significantly after thelong-tern continuous cropping, which means that the disease-suppressive soil has formed bythis management. For sustaining this hypothesis, from the relationship among continuouscropping of soybean, pathogenic microbes and suppressive microbes, the pathogenicmicroorganism and their population abundance, diversity and pathogenic differentiation werestudied using the traditional isolating, counting and morphological identification methodscombined with sequencing analysis, real-time PCR and denaturing gradient gel electrophoresis(DGGE) techniques. Meanwhile, according to the analysis of soil microbial population in long-tern continuous cropping, the suppressive microbes and their population abundance anddiversity in three rotation systems were explored.Compared with3years continuous cropping, the situation of root length, ground freshweight and underground fresh weight were improved significantly by20years measure(P<0.05), but not for plant height (P>0.05). The attack of soybean root rot was also milder than3years treatment. There were84strains isolated from the suffering root of soybean classifiedinto7genuses. However, only some strains from Fusarium genus could cause the soybean rootrot. Using the sequencing analysis of EF-1α and DGGE method, the Fusarium speciescolonizing on the soybean roots were confirmed, including F. avenaceum, F. culmorum, F.equiseti, F. graminearum, F. solani, F. oxysproum, F. verticillioides and a relative specie with F.oxysproum. Through the test of pathogenicity, the pathogenic species were confirmed,including F. avenaceum, F. graminearum, F. oxysproum, and F. equiseti, F. solani and F.verticillioides were the nonpathogenic ones.Combined dilution plate method and real-time PCR, the genomic DNA quality and theCFU of soil Fusarium population in20years continuous cropping treatment were4.5ng pergram of soil and2.7×10~4per gram of soil, which significantly lower than that in3yearstreatment. Meanwhile, the result of identification and DGGE showed that the dominance of F.oxysproum species in20years treatment had decreased from the level of3years treatment.According to the isolation frequency of Fusarium spp., the diversity, evenness and dominanceof Fusarium population in20years continuous cropping had been improved than3years one.From the result of principal component analysis (PCA) and cluster analysis, the Fusariumpopulation in20years treatment had been altered by decreasing the abundance of certain dominant species.Using all above method to analyze the microbial population in long-tern continuouscropping field, it was shown that the abundance and structure of bacterial population was notinfluenced. There were not any specific microbes to select, and just pick the dominant speciesPseudomonas fluorescens. However, the CFU of fungal population was decreased, the diversitywas increased and the structure was altered thoroughly. Using the DGGE method, the specificmicrobes had been selected, which were Verticilium chlamydosporium and Trichoderma spp..Meanwhile, the bacterial to fungal ratio of CFU and genomic DNA quality in3years treatmentwere88.9(±22.3)and1.5(±0.8)respectively, both of which lower than that in20yearstreatment.According to the test of suppression capability, there were not any suppressive strains in44P. fluorescens and8V. chlamydosporium isolates. However, there were32suppressiveisolates in all44Trichoderma strains. Except one T. viride strain,23strains isolated from3years treatment owned capability that inhibited the pathogenic Fusarium, but there were just8strains among all18ones from20years treatment represented suppressive ability. T.harzianum was the dominant species in3years treatment, which act the superparasitism as thesuppressive mechanism. T. virens was the dominant species in20years treatment, which usingthe secondary metabolic production as the suppressive mechanism. Meanwhile, all strains from3years treatment were slightly better than that from20years treatment as a biological controlagent for soybean root rot disease, but all Trichoderma strains from20years treatment alsopresented growth promotion for soybean plants. The linear relation between Trichoderma spp.and Fusarium spp. was established by the result of CFU and genomic DNA quality. T.harzianum and T. virens were the dominant species in3years treatment, but this dominance ofT. harzianum had decreased after20years continuous cropping.