The microbiota of 84 different agricultural soils were transferred to separate samples of a γ irradiation-sterilized field soil enriched with potato periderm, and the resulting soils were assayed for biological suppressiveness to Phytophthora erythroseptica and their effect on zoospore production. The 13 most suppressive soil samples, which reduced zoospore production by 14 to 93% and disease severity on tubers by 6 to 21%, were used to isolate 279 organisms. Fourteen strains that reduce pink rot infections in preliminary tests were selected for further study. Six bacterial strains that reduced the severity of disease (P ≤ 0.05, Fischer's protected least significant difference) in subsequent tests were identified as Bacillus simplex (three strains), Pantoea agglomerans, Pseudomonas koreensis, and P. lini. Relative performance indices (RPIs) for biocontrol efficacy and for each of four kinetic parameters, including total colony-forming units (CFUmax), biomass production values (DWmax), cell production after 8 h (OD8), and time of recovery from oxygen depletion (DT) were calculated for each strain. Overall RPIEff,Kin values for each strain then were calculated using strain RPI values for both efficacy (RPIEff) and kinetics (RPIKin). Strains with the highest RPIEff,Kin possess the best biocontrol efficacy of the strains tested and liquid culture growth characteristics that suggest commercial development potential.
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