2010 Annual Report
1a.Objectives (from AD-416)
Evaluate the impact of soil environmental factors on the behavior (including disease suppression and colonization of plant surfaces) of biological control agents (BCAs). Evaluate the impact of interactions resulting from the combination of BCAs with other biologically or chemically based control measures on the performance of BCAs. Develop strategies to enhance compatibility between BCAs combined with other BCAs or cover crops in biologically based disease control strategies directed at controlling Pythium ultimum, Phytophthora capsici, and Meloidogyne incognita on cucumber and pepper. Develop strategies for combining chemical pesticides with biologically based disease control methods in integrated pest control strategies directed at controlling P. ultimum, P. capsici, and M. incognita on cucumber and pepper.
1b.Approach (from AD-416)
Use basic microbiology techniques to determine soil factors that negatively impact performance of biological control agents. Use techniques from molecular biology, biochemistry, and basic microbiology to identify genes to use as reporters for environmental conditions that negatively impact biological control performance. Multi-disciplinary collaboration in growth chamber and greenhouse studies will determine impacts of cover crops and chemical controls on performance of biological control agents.
Five subobjectives of the Project Plan were scheduled to be worked on during this Annual Report cycle. For one of these subobjectives, microarray experiments were conducted in collaboration with scientists at USDA-ARS, Corvallis and at Rutgers University to determine the impact of two spermosphere environments on expression of genes within the biological control bacterium Pseudomonas fluorescens; including those genes involved in disease suppression. Microarray data was obtained but was limited to that obtained from this bacterium growing on cucumber seeds germinating on filter paper. Pilot experiments are currently being conducted to develop In Vitro Expression Technology for determination of the impact of spermosphere environments on expression of biological control and other genes in the plant-beneficial bacterium Enterobacter cloacae. The new approach should allow detection of a limited number of genes expressed in E. cloacae while growing on crop seeds germinating in natural soil. A second subobjective scheduled to be worked on during this Annual Report cycle concerned the identification of genes involved in stress response pathways and the analysis of their expression in various soil environments. Genes functioning in the RpoE stress response pathway were demonstrated to be important for Enterobacter cloacae while colonizing soil and root environments. The third and fourth subobjectives concerned the analysis of combinations of biological control agents with cover crops with regard to suppression of disease caused by the soilborne pathogens Pythium ultimum and Phytophthora capsici. Combinations of sorghum sudan grass with Enterobacter cloacae effectively suppressed Pythium ultimum disease on cucumber. There was evidence that combining this biological control bacterium with the sorghum sudan grass cover crop resulted in an additive effect in suppression of this pathogen on cucumber. None of the cover crops tested show suppression of Phytophthora capsici on pepper. Work on the fifth subobjective has been initiated. Cover crops have been prepared for analysis of their impact on colonization of cucumber by biological control bacteria. A Trust Agreement was established with Smithers Oasis Corp., Cuyahoga Falls, OH in July, 2006. One objective of the Trust Agreement was to evaluate the compatibility of Smithers Oasis foam products with select beneficial bacteria and fungi. It was found that Trichoderma spp. and Gram-negative bacteria persist in Smithers Oasis grower foam for periods of at least six weeks. A second objective of the Trust Agreement was to test the ability of biological control agents to suppress soil-borne plant pathogens when crops were grown in Smithers Oasis grower foam. Five Trichoderma isolates were isolated that suppress Phytophthora capsici on bell pepper, a pathosystem compatible with Smithers Oasis foam products. These Trichoderma isolates failed to suppress P. capsici on pepper when pepper was grown in the Smithers Oasis foam. These same five Trichoderma isolates effectively suppressed Pythium ultimum damping-off of cucumber when cucumber was grown in Smithers Oasis foam.
Complex combinations of nutrients in exudates released from seeds and roots cause stress in plant-beneficial bacteria. Stress-inducing environments occupied by plant-beneficial bacteria have a negative impact on performance by these bacteria. These stress-inducing environments need to be identified and their impacts on plant-beneficial bacteria determined so that strategies can be developed to minimize the negative impacts of these environments. Using a mutant of the plant-beneficial bacterium Enterobacter cloacae it was shown that the RpoE stress response pathway is critical for biological control of soilborne pathogens, colonization of plant roots, and growth on complex mixtures of nutrients found in exudates from roots. This is the first time that the RpoE stress response pathway was demonstrated to be important in growth on root exudates and by extension the first time that it was demonstrated that complex mixtures of nutrients in root exudates cause bacterial stress. This information will be useful to scientists devising strategies to enhance performance of biological control bacteria for suppression of soilborne plant diseases.
Baker, C.J., Owens, R.A., Whitaker, B.D., Mock, N.M., Deahl, K.L., Roberts, D.P., Averyanov, A.A. 2010. Effect of viroid infection on the dynamics of phenolic metabolites in the apoplast of tomato. Physiological and Molecular Plant Pathology. 74:214-220.