| Recent Research Results and Products |
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RECENT RESEARCH RESULTS AND PRODUCTSGlomalin is a unique and abundant component of soil organic matter. Glomalin, a glycoprotein produced by arbuscular mycorrhizal (AM) fungi, was discovered in the early 1990s. Glomalin appears to be a complex structure bound together by hydrophobic interactions with a consistent structure across soils. The hydrophobic component of glomalin may allow this glycoprotein to coat AM hyphae to reduce solute loss. Arbuscular mycorrhizal fungi colonize 80% of vascular plant species and are found worldwide in almost every soil. As AM fungal hyphae degrade, glomalin sloughs off onto soil particles providing a hydrophobic coating for stabilization. In temperate soils, glomalin amounts vary from 2 to 15 mg/g soil. Recent work showed that glomalin is a unique and major component of extractable organic matter. A comparison was made of components extracted by classical techniques used in humic acid (HA) research and techniques used to extract glomalin. Weights and 1NMR spectra of HA and glomalin were compared in eight soils representing different geographic regions of the US. By gravimetric and carbon weight, glomalin contributed more to soil organic matter than did humic acid. 1NMR spectra showed a unique structure on glomalin that is not present on humic acids. On average, glomalin comprised 27 percent of the total soil carbon for soils from the four geographic regions of the US. This information will change the way the soil organic matter is investigated and the way that management of soils will be pursued to maintain or increase stability and productivity. Glomalin levels can be monitored to determine effects of management practices on soil quality. Contact: Dr. Sara Wright. Release of the first forage soybean cultivars developed by the Agricultural Research Service. Conventional plant breeding techniques were used to develop three forage soybean cultivars with exceptionally tall (up to six feet) stature and lodging resistance: Donegal, Derry, and Tyrone. Donegal was released for use in the Northeast, Derry for use in the northern Midwest, and Tyrone for use in the southern U.S. Acreage planted to these cultivars has grown rapidly and it is estimated that they were grown on 65,000 acres in the U.S. in 2002. A new multi-use soybean cultivar 'Tara' also was released that produces more crop residue. Use of this new tall-growing soybean cultivar will reduce soil erosion as well as establish the principal of using high biomass producing cultivars to reduce erosion. In addition, to fill the need for the rapidly growing demand for organically grown vegetable soybeans, a new vegetable soybean cultivar named 'Moon Cake' that can grow to heights of over six feet and competes better than available cultivars with weeds in organic production systems was released. Contact: Dr. Tom Devine. Developed a cover crop-based system for tomato production in subtropical Florida. After four years of research, a biologically-based alternative system was developed using the nematode-resistant cover crops sunn hemp (Crotalaria junica cv. Tropic Sun), cowpea (Vigna unguiculata cv. Iron Clay), and velvetbean (Mucuna deeringiana). Results of two years on tomatoes planted in nematode infested soils in south Florida show that the alternative system yielded equal to or higher than the standard system based on methyl bromide (MeBr) applied as a pre-plant soil fumigant. Yield increases plus savings on MeBr and fertilizers resulted in an average net return of $1,400/ha over the MeBr-based production system. In addition to suppressing nematodes, the alternative system r educed soil erosion, improved soil fertility, and protected t he environmentally fragile agroecosystem of the Everglades from harmful pesticides. Contact: Dr. Aref Abdul-Baki. Optimum planting dates and performance of early hairy vetch cultivars. Hairy vetch has become an important cover crop for sustainable production of crops but adapted cultivars are needed in various regions of the United States. Research was conducted at Beltsville and Salisbury, MD, and Freeville, NY, to evaluate the performance of new cultivars developed at Auburn University and common hairy vetch cultivars under northeastern conditions. Results showed that the Auburn cultivars may be useful to mid-Atlantic producers who need earlier flowering cultivars but that they are insufficiently winter hardy for use in New York. A simple model of growth and development based on growing degree-days could be used to determine optimum hairy vetch planting dates throughout the northeast. This research should provide growers and extension personnel with information on the optimum cultivars and planting dates for growing a hairy vetch cover crop in the northeastern states. Publication: Contact: Dr. John Teasdale. Weed seedbank dynamics in organic rotations. Crop rotation is an important means of reducing weed seed populations in soil that can lead to additional weed problems in future years. Three organic farming rotations were compared in the long-term Farming Systems Project to assess how populations of weed seeds changed over a six year period since the comparison began in 1996. Rotations composed of only the summer annual crops corn and soybeans had high soil seed populations of summer annual weeds. Adding a winter annual wheat crop or a perennial hay crop reduced seed populations of the most troublesome summer annual weeds. Although substantial increases in weed seeds in the soil could occur following years of poor weed control in corn or soybean, substantial decreases could occur if weeds were controlled using diverse rotational crops. Sequences that began with corn and soybeans had higher weed seed populations than sequences that began with hay. Lower weed seed populations in soil often led to better weed control in the succeeding corn crop reinforcing that maintaining low weed seed populations is important for organic farming.
Publications: Contact: Dr. John Teasdale. Soil type is more important than plant species in determining rhizosphere microbial communities. Many biocontrol bacteria need to colonize the spermosphere or rhizosphere in order to function. This requires successful competition with the indigenous microbial community. Identification of the factors controlling microbial community structure is a necessary step in developing novel strategies to maximize colonization and improve biocontrol of fungal pathogens such as Fusarium, Verticillium, and Pythium. Rapidly growing aerobic heterotrophic bacteria, which make up a very small percentage of the total community, were affected by plant species, but the soil type was far more important than plant species in determining the composition of the total microbial community. This work demonstrated that the soil is more important than the plant in determining soil and rhizosphere microbial communities, and that the classic view of the 'rhizosphere effect' may be exaggerated. This means that colonization studies with potential biocontrol agents need to be carried out in a wide variety of soils, and that efforts to improve colonization need to focus as much on soils and indigenous communities as on the plant species to be colonized. This research also compared two widely used assays for microbial communities, and demonstrated that the substrate utilization assay may not be measuring community function, as previously believed, but instead is measuring the community structure of culturable aerobic heterotrophic bacteria. Contact: Dr. Jeff Buyer. Identified genes that encode key enzymes that control root colonization by biocontrol organisms. Colonization of subterranean portions of plants by beneficial microbes is thought to be essential for disease suppression in many biocontrol interactions. Unfortunately colonization is a poorly understood process. The genes pfkA, rpiA, sdhA, and degS were demonstrated to be important for colonization of cucumber seeds and/or roots by the plant-beneficial bacterium Enterobacter cloacae. The genes pfkA, rpiA, and sdhA encode key enzymes in glycolysis, the pentose phosphate pathway, and the tricarboxylic acid cycle, respectively. This established these pathways and the catabolism of carbohydrates and other reduced carbon compounds as important substrates for colonization of plant surfaces. The gene degS functions in stress responses. In addition, a method was developed that allows for the rapid determination of the spatial distribution of plant-beneficial bacteria during colonization of the rhizosphere. Prior methods were extremely labor-intensive making these studies prohibitive to perform. Contact: Dr. Dan Roberts. Identified genes required for tagetitoxin, a phytotoxin produced by P. syringae pv. tagetis, a potential biological control agent of Canada thistle.Pseudomonas syringae pv. tagetis, a pathogen being developed as a biological control agent of Canada thistle (Cirsium arvense), produces tagetitoxin, a phytotoxin that prevents the development of chloroplasts in meristematic tissue and which results in apical chlorosis or white top in infected plants. Using Tn5 mutagenesis, two genes required for tagetitoxin production were identified. Predicted proteins of these genes have homology with the TonB system, an iron transport system, and an asparagine synthase. PCR protocols based on the DNA sequences of these genes were developed that allow P. syringae pv. tagetis to be distinguished from other P. syringae pathovars. This is the first report on genes related to tagetitoxin production. Using the PCR protocols developed from this work, two newly described Pseudomonas syringae strains capable of producing apical chlorosis were determined not to be P. syringae pv. tagetis strains. The PCR protocol may also be useful in monitoring the pathogen in target plants and in identifying vectors of the disease. Contact: Dr. John Lydon.
Mite as a Potential Biocontrol Agent For additional info, email Dr. John Lydon. Mites Get Frozen, Photographed, and Identified (ARS Magazine article) Discovered the ability of eastern gamagrass to tolerate acid, compact soils. Mechanical impedance, low pH, aluminum toxicity, and waterlogging of soils are important limitations to successful culture of crop plants. It has been estimated that 39% of the soils in the mid-Atlantic region have root restriction layers within 50-100 cm of the surface. Species are needed that are adapted to such sites. Eastern gamagrass is a perennial, warm-season grass, native to eastern North America that can be used as a forage crop, vegetative hedges, and to ameliorate marginal soils. Roots of eastern gamagrass were shown to penetrate an aluminum toxic soil while those of sordan, a hybrid of sorghum and sudan grass, were unable to do so. In research conducted over a four-year period, eastern gamagrass was able to produce relatively high levels of biomass despite adverse stress imposed by shallow top soil, low pH, high bulk density and severe deficits in soil moisture during three of the four years. These findings demonstrate the ability of eastern gamagrass to tolerate and even flourish on an acid, aluminum-toxic, compact soil. These findings have shown that warm season grasses have high forage value, can serve as a grass hedge to reduce the loss of soil and nutrients to adjacent streams, and can be used for reclamation of marginal lands. Contact: Dr. Don Krizek. Long-term economic and environmental analyses demonstrate the potential strengths and weaknesses of sustainable cropping systems. A long-term comparison of cropping systems has been conducted on the South Farm Demonstration site since 1994. Based on results from this site and a 60-year simulation using cropping systems models, the tradeoffs between profitability, economic risks, and environmental hazards were determined. Results indicated that a cover crop-based system was the most profitable with minimal levels of erosion and herbicide losses, but high variability of profits would make this system less attractive to risk-averse farmers. Reduced-tillage organic rotations were more attractive to risk-averse farmers because of lower variability of profits as well as low erosion and absence of pesticide usage. The biggest hazard to sustainability was nutrient losses; all systems had nitrogen runoff that exceeded threshold levels in at least two-thirds of the simulation years and systems that were lowest in nitrogen runoff were highest in phosphorus losses. This research demonstrated that control of nutrient losses represents a major challenge to the design of sustainable cropping systems. Publication: Contact: Dr. Yao-Chi Lu. Vinegar as an herbicide. Former Research Associate Jay Radhakrishnan conceived of and demonstrated the efficacy of vinegar for controlling weeds. Results indicated that vinegar can kill several important weed species at several growth stages. Vinegar at 10, 15 or 20 % acetic acid concentration provided 80-100 percent kill of selected annual weeds, including giant foxtail up to 3 inches in height, common lambsquarters up to 5 inches, smooth pigweed up to 6 inches, and velvetleaf up to 9 inches. Control of annual weeds with vinegar at the 5 % acetic acid concentration (most products sold in grocery stores contain 5% acetic acid) was variable. Canada thistle shoots were highly susceptible with 100 percent kill by 5 % vinegar, however, there was re-growth from roots. Established perennial weeds usually will not be killed by vinegar. In addition, weeds must be sprayed to complete foliage wetness; typical volumes used for broadcast application of herbicides are not effective with vinegar. Potential uses of vinegar for controlling weeds include spot treatments of weed patches between rows of plastic mulch or other areas where contact with crop plants can be avoided, control of unwanted vegetation along roadsides and range lands, and control of weeds by homeowners around yards, brick walls and cracks in pavement. Contact: Dr. Ben Coffman.
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