A Screening Strategy And Evaluation Of Fungal Biocontrol Agents Against Cotton Verticillium Wilt

Verticillium wilt caused by Verticillium dahliae Kleb is the most destructive disease of cotton and hard to control.For biocontrol of this disease,105 antagonists were screened from 373 fungal strains isolated from different origins of cotton fields(healthy plants in healthy fields,healthy plants and unhealthy plants in infected fields,their rhizosphere and bulk soils).In order to establish a fast and effective screening strategy of potential biocontrol agents,each antagonist was given an assessment value of biocontrol and plant growth promotion efficiency according to antagonistic experiments in vitro,production of protease,cellulose and chitinase activities,as well as abilities of phosphate-solubilizing and nitrogen-fixating.Thirty-three antagonists were selected for the first biocontrol efficiency and growth promotion experiments in greenhouse for validating this evaluation system. Their biocontrol efficiencies were -64.20～69.78%and biomass increases were -29.16～88.62%.Then 13 strains with positive biocontrol efficiencies were picked for further two times greenhouse experiments and were identified according to their ITS sequences.For the new screening strategy,we found more antagonists could be obtained from endophytic cottons in the infected field than from other origins.Excluding strains with minus biocontrol efficiencies,assessment values of all other strains were found basically coherent with the biocontrol efficiencies in greenhouse experiments,which indicated that the new screening strategy was effective for screening potential biocontrol agents.At the same time,several potential biocontrol agents were obtained through this strategy.The disease indexes of cotton treated with strains By125(Fusarium oxysporum) and Bx247 (Nectria haematococca) revealed the significance compared with control group(infected with pathogen only) at P＜0.05,and their biocontrol efficiencies reached 69.03%and 69.78%,with biomass increases of 34.67%and 62.63%respectively.By247(Nectria haematococca),By125(Fusarium oxysporum) and By231(Phomopsi sp.) whose biocontrol efficiencies were 69.78%,69.03%and 63.43%respectively in the greenhouse experiment were also tested in field.The results showed their biocontrol efficiencies in flower initial stage were 33.25%,30.12%and 29.64%respectively,with biomass increases of 13.60%, 13.00%and 10.75%respectively,(There was no harvesting because of the cold weather). It showed that the application method to open field should be modified.This is the first report of using living fungal preparation of Phomopsis sp.(By231) as potential biocontrol agent against Verticillium wilt and promoting plant growth of cotton.Employing Hayward's classification criterion of Ralstonia solanacearum,181 strains of R.solanacearum isolated from eight kinds of host plants in China were classified.The results revealed that the main biochemical variant of R.solanacearum were biovar 3 (54.7%) and 4(33.7%).In contrast,the proportion of biovar 2,5 and 3-1 were 3.9%,1.7% and 6.1%respectively.Biovar 1 has not been found in this research.