1a. Objectives (from AD-416):
1) Develop integrated multi-tactic weed management programs which will include cover crops, cultural practices, and bio-herbicides for reduced-tillage production systems. 2) Determine the ecological mechanisms by which crop and soil management impact crop and weed establishment and growth processes.
1b. Approach (from AD-416):
A multi-tactic approach will be developed for reduced-tillage corn production that combines the optimum hairy vetch-rye cover crop mixture for suppressing weeds with poultry litter injection that supplements nitrogen required by the crop, but localizes it in space and time to minimize utilization by weeds. Soil properties, mulch characteristics, and weed sizes will be determined that optimize the postemergence destruction of weeds with a high-residue cultivator, minimize disturbance of the surface residue, and maximize soybean yield. Methods will be developed to augment and purify naturally-produced bacterial phytotoxins and test their efficacy as a postemergence herbicide. Basic research will be conducted to increase our understanding of weed suppression by cover crop mulches in relation to potentially allelopathic compounds leached into the soil and their duration of soil activity. Basic research also will be conducted to determine the effect of soil organic matter on mitigating the competitive effects of weeds on crops.
3. Progress Report:
A component experiment was completed that determines the optimal cover crop species composition (cereal:legume) and poultry litter rate and placement method that minimize weed competition and maximize nitrogen use efficiency in corn. The second year’s field experiment was completed and crop, soil, and weed metrics were collected. This experiment has attracted a broad multi-disciplinary team within ARS as well as land grant universities and has been included in several grant applications. Two University of Maryland M.S. students have selected this experiment for their thesis and have initiated two tangential projects within this study. The first thesis will focus on assessing N and C spatio-temporal dynamics while the other student will assess greenhouse gas emissions within the project. A report on the role of the small regulatory protein RsmA in production of the phytotoxins syringomycin, phaseolotoxin, and tabtoxin, and other virulence factors, was published in a peer-reviewed journal. A decision was made to increase the scope of study regarding the regulatory network involved in the production of the phytotoxin tagetitoxin by the potential weed biological control agent Pseudomonas syringae pv. tagetis. This required that resources be shifted away from studies directed at determining the efficacy of natural herbicides towards this work. Experiments were initiated with an ARS collaborator at the Beltsville Agricultural Research Center to finalize sequencing of the P. syringae pv. tagetis genome. This will allow mapping of genes mutated in mutants no longer capable of production of tagetitoxin. A collection of such mutants was developed previously by scientists in the Sustainable Agricultural Systems Laboratory. These sequencing experiments will potentially identify genes involved in regulation of tagetitoxin and also identify genes involved in the synthesis of this phytotoxin. Experiments were continued to determine the role of components in the Gac/Rsm regulon in production of tagetitoxin. Plasmids have been created to overexpress five rsmX genes in P.syringae pv. tagetis. Creation of plasmids to demonstrate the roles of rsmY and rsmZ continues to be a challenge. Results from previous allelopathy research have provided new insights on the activity of allelochemicals in soil. For example, solution-phase availability was found to be critical and may need more careful evaluation. Consequently, we deferred initiation of microplot and allelopathy research designated in the project plan and have begun discussions on a more appropriate line of research to begin next year. A two-phase, two year greenhouse pot experiment was completed. The first trial examined the effects of soil from long-term conventional and organic cropping systems on the functional densities required for nitrogen utilization of crop and weed species. Results from the first experiment were used to design the subsequent weed-crop competition experiment using the same soils. Both trials have been completed and a manuscript is in preparation.
Kong, H., Roberts, D.P., Patterson, C.D., Kuehn, S., Heeb, S., Lakshman, D.K., Lydon, J. 2012. Effect of overexpressing rsmA from Pseudomonas aeruginosa on virulence of select phytotoxin-producing strains of P. syringae. Phytopathology. 102:575-587.