Submitted to: Applied and Environmental Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/20/2003
Publication Date: 7/20/2003
Citation: Ownley, B.H., Duffy, B.K., Weller, D.M. Identification and manipulation of soil properties to improve the biological control performance of phenazine-producing pseudomonas fluorrescens. Applied and Environmental Microbiology. 2003. v. 69. p. 3333-3343. Interpretive Summary: Take-all caused by the soil fungus Gaeumannomyces graminis is the most important root disease of wheat worldwide. Strains of Pseudomonas fluorescens, which produce the antifungal, biocontrol metabolite phenazine-1-carboxylic acid (PCA), provide significant suppression of take-all when applied as a seed treatment. However, the level of control provided by the introduced bacteria can be inconsistent, and varies from field to field. We analyzed the characteristics of representative wheat field soils from the Pacific Northwest, and developed a regression model that predicts soils where PCA-producing biocontrol strain Pseudomonas 2-79 shows good and poor performance. This work provides new insight into the reasons for inconsistent performance by biocontrol agents, and demonstrates that consistent high level disease control can be achieved if biocontrol agents are paired with soils that are most conducive to their growth and activity.
Technical Abstract: Pseudomonas fluorescens 2-79RN10 protects wheat against take-all disease caused by Gaeumannomyces graminis var. tritici; however, the level of protection in the field varies from site to site. Identification of soil factors that exert the greatest influence on disease suppression is essential to improving biocontrol. In order to assess the relative importance of 28 soil properties on take-all suppression, seeds were treated with strain 2-79RN10 (which produces phenazine-1-carboxylate [PCA+]) or a series of mutants with PCA+ and PCA- phenotypes. Bacterized seeds were planted in 10 soils, representative of the wheat-growing region in the Pacific Northwest. Sixteen soil properties were correlated with disease suppression. Biocontrol activity of PCA+ strains was positively correlated with ammonium-nitrogen, percent sand, soil pH, sodium (extractable and soluble), sulfate-sulfur, and zinc. In contrast, biocontrol was negatively correlated with cation-exchange capacity (CEC), exchangeable acidity, iron, manganese, percent clay, percent organic matter (OM), percent silt, total carbon, and total nitrogen. Principal component factor analysis of the 16 soil properties identified a three-component solution that accounted for 87 percent of the variance in disease rating (biocontrol). A model was identified with step-wise regression analysis (R2 = 0.96; Cp statistic = 6.17) that included six key soil properties: ammonium-nitrogen, CEC, iron, percent silt, soil pH, and zinc. As predicted by our regression model, the biocontrol activity of 2-79RN10 was improved by amending a soil low in Zn with 50µg of zinc-EDTA/g of soil. We then investigated the negative correlation of OM with disease suppression and found that addition of OM (as wheat straw) at rates typical of high-OM soils significantly reduced biocontrol activity of 2-79RN10.