- Agronomic: canola, rapeseed
- Fruits: berries (strawberries)
- Vegetables: tomatoes
- Crop Production: cover crops, tissue analysis
- Education and Training: demonstration, extension, on-farm/ranch research
- Pest Management: botanical pesticides, economic threshold, mulching - plastic, soil solarization
- Production Systems: agroecosystems
- Soil Management: organic matter, soil analysis, soil quality/health
There is an accumulation of experimental data supporting the concept that many classes of biological pest management are effective in controlling diseases caused by soilborne pathogens. The mode of action of many of these treatments has been described. The volatile compounds released from Brassicas are known to release biocidal compounds that are able to suppress soilborne pathogens when incorporated into the soil (Kirkegaard, 1998). Composted amendments can introduce a varied microflora into soil ecology, including many biocontrol agents, while providing a suitable substrate to support their growth (Hoitink, 1999). Solarization has also been shown to reduce pathogen propagule numbers by elevating soil temperatures to lethal levels and contribute to effective disease control at sub-lethal temperatures (Katan, 1991).
Sustainable control of soilborne phytopathogenic fungi is likely to be achieved through the enhancement of biologically based methods via the integration of multiple techniques (Cook, 1990). Research has indicated that a pre-plant combination of biofumigation with composted organic amendments increases the efficacy of both treatments (Cook 1993, Lyons 2001). Even sub-lethal doses of biofumigants can act to weaken pathogen propagules making them more susceptible to the actions of the microbial antagonists that can be delivered through compost applications. Similarly, the heating of soilborne sclerotia via solarization increases their chance of colonization by bacteria (Lifshitz, 1983). It is clear that a fundamental attribute of an effective integrated organic system is its ability to, firstly, decrease pathogen numbers in the soil and then establish a soil microecology suppressive to pathogens.
The viability of biofumigation to control pathogens has been investigated for many years. Two projects recently funded through the Southern SARE graduate research program successfully developed an integrated production system that combines an enhanced form of biofumigation with the many benefits of composts. The studies conducted at The University of Tennessee by doctoral students Martin Lyons and Stephanie Harvey showed that mustard seed meal has extremely high concentrations of isothiocyanates (ITCs). The seed meal is also a significant source of nitrogen and other nutrients. When incorporated into the soil, ITCs act as effective biofumigants, reducing populations of pathogenic fungal species (Sclerotium, Rhizoctonia, and Phytium), nematodes, weeds, and certain insect species (Charron and Sams, 1999).
Biofumigation is the use of volatile plant chemicals for control of soilborne pests. These products have been shown to suppress the pathogens Botrytis cinerea, R. solani, F. oxysporum, Didymella lycopersici, and Cladosporium fulvum (Urbasch, 1984). Although the fungicidal properties of ITCs have been identified since at least 1937 (Walker et al., 1937), only recently has the practice of using Brassicas for pest control been termed “biofumigation” (Angus et al., 1994). In our own studies, we found that the volatiles from several Brassica species suppressed the growth of the tomato pathogens P. ultimum, R. solani, and S. Rolfsii (Charron and Sams, 1998; Charron and Sams, 1999: Harvey, Hannahan, and Sams, 2002)
The biocidal activity of Brassicas against fungal pathogens, nematodes, weeds, and insects is frequently attributed to ITCs from Brassica tissues (Delaquis and Mazza, 1995). ITCs are effective, broad-spectrum pesticides (Mithen et al., 1986; Isshiki et al., 1992), and substantial quantities of them can be produced for field application. Research has shown that Black mustard (B. nigra L. W. Koch) and Indian mustard (B. juncea L. Czern and Coss.) produce high levels of ITC (Tollsten and Bergström, 1988) and could be utilized in a biofumigation cropping system.
Control of V. dahliae by Brassica residues has also been shown. The number of V. dahliae propagules in a 36-m2 plot incorporated with 200 kg of chopped broccoli (B. oleracea var. italica) were lower than in control plots and were comparable to plots fumigated with methyl bromide + chloropicrin (Subbarao et al., 1994).
Since 1999 field trials have been conducted at Knoxville Experiment Station. Significant decreases in the incidence of Southern Blight of tomato were recorded, as well as increases in fruit yield, when integrating biofumigation into a sustainable production system. The research yielded important knowledge pertaining to the development of the appropriate production methods utilizing biofumigation as a management technique. Both Brassica cover crops and mustard seed meal incorporations have been used. The ability to amend production soils with the spreadable meal, in conjunction with its high ITC content (3-4 x leaf tissue), give this enhanced biofumigation technique great potential. Problems associated with growing cover crops such as variable stands and weather complications are avoided when using the meal. As a fully organic product, pre-plant mustard meal applications give the grower superior ability to control soilborne pests.
Composts and Biological Control
Composts improve plant health and productivity due to enhanced soil nutrient levels and improved physiochemical properties (increased bulk density, porosity, aeration) and increased water-holding capacity (Corti 1998). Disease suppression due to the advantageous changes to the soil microbial composition, including increases in the populations of known biological control agents (Trichoderma, Bacillus, and Pseudomonas spp.), has been noted by many researchers (Hoitink 1999, Lyons 2001). Composting is an ecologically sound way to recycle organic farm wastes, such as animal manures and plant debris. It avoids the environmental hazards associated with burning, burying, or spreading these agricultural by-products on fields. These methods are not sustainable and can lead to problems such as increased particulate matter in the air, leaching and ground water pollution, and toxic excesses of phosphorous and nitrogenous compounds in the soil.
Potential problems with compost as a pre-plant treatment include using a product that has not been composted properly and, hence, is not mature. Compost testing is to be encouraged. With the assistance of extension agents and university resources maturity levels can be gauged by measuring C:N ratios and pathogen levels assessed via assays. Feedback from this type of testing can direct more effective composting methods.
Many researchers throughout the world have investigated the relationship between the addition of composts to soil and plant health. The pre-plant application of composts has many advantages. They have the ability to provide biological control of plant diseases while simultaneously improving physical soil properties and micronutritional levels (Corti, 1998). The composted material used is often derived from agricultural waste products and, hence, serves to “recycle” these voluminous compounds. Composted soil amendments act to create a suppressive environment in the soil that can biologically control soilborne pathogens through microbial competition, antagonism, antibiosis, hyperparasitism, and possibly induce systemic resistance in plants (Hoitink, 1994). In particular two types of composts will be studied, poultry waste compost (PWC) and spent mushroom substrate (SMS).
The three phase composting process ultimately leads to the recolonization of the substrate with mesophilic microflora and microfauna. (Hoitink, 1986). Microorganisms commonly found in composts include: Bacillus spp., Enterobacter spp., Pseudomonas spp. Streptomyces spp., Trichoderma spp., and Gliocladium spp. (Hoitink, 1994). Some of these species are recognized to exhibit properties of biological control.
Biological control of plant pathogens has been observed and studied for many years. As defined by Baker (1987), biocontrol is “the action of parasites, predators, or pathogens in maintaining another organism’s population at a lower average than would occur in their absence.”
Tomato Production and Blight diseases as a Model System
The use of raised-bed plasticulture methods for fruit and vegetable production is becoming increasingly prevalent within the United States. Advantages include increased crop performance by conserving moisture and nutrients, stabilizing soil temperature, reducing some diseases, reducing or eliminating weeds, and increasing early harvest yields (Jaworski, 1981). Hence, we aim to adapt this commonly used production method for commercial organic agricultural production. Often grown without rotation, tomato fields can develop high pathogen inoculum densities. Southern Blight, caused by the phytopathogenic fungus Sclerotium rolfsii, represents one of the major disease threats to tomato crops (both organic and conventional) in the southeast United States. However, this pathogen is also a threat to many other crops. Significant economic losses have been attributed to Southern Blight in tomato, green beans, cantaloupe, peppers, and potatoes grown in predominantly warm, humid seasons. The disease spreads most effectively between 25-30C. These factors make the investigation of pre-plant soil treatments for organic tomato production under pressure from Southern Blight a suitable choice as a model system for our investigations.
Various control methods have traditionally been utilized to lessen the incidence of Southern Blight in commercial vegetable fields. Control by solar heating has been successful in Israel. Studies by Elad and Katan (1980) have demonstrated that 2-4 week applications during the summer were able to reduce sclerotial numbers and limit disease. Additionally, these researchers demonstrated that combining solar heating with applications of Trichoderma harzianum was more effective than either treatment alone. Punja (1985) contended that by altering the composition or activities of soil microflora, including the addition of biocontrol agents such as antagonistic microorganisms, that there is a potential for disease control. T. harzianum, which has been described as a destructive mycoparasite that competes aggressively against other soil fungi, is acknowledged to suppress Southern Blight when introduced to field soils (Jenkins, 1986). The growth of the biological control agent T. harzianum is enhanced with solar heating (Jenkins, 1986).
The Growth of Organic Agriculture
In October of 2002 the USDA fully implemented the National Organic Program Standards (NOPS). The result is a clarification of the certification for organically farmed produce. Consumers will be able to easily identify produce grown via organic methods and have confidence in their purchases. Even though there has been a steady increase in demand for organic produce, it is likely that consumption will increase markedly because of the certification.
Already identified as being the fastest growing agricultural sector, these recent changes represent an opportunity for regional farmers to view organic farming as a potentially profitable alternative to traditional, non-sustainable crop production (Koenig, 2002). Successful modern organic farming strives to be farmer-oriented, multidisciplinary, system-oriented, with a whole-farm perspective (Lockeretz, 2000). The USDA has identified certain non-synthetic biological disease management inputs as being necessary for aggressive pathogen control. The use of botanical, or plant-derived compounds, are recognized. Biofumigation is such a method.
The USDA standards also recognize the benefits of applying mature, well-produced composts in preference to raw animal manures. In order to assist organic growers with the development of reliable, high-yielding production systems research is required on the effective utilization of biofumigation, composting and other sustainable farming methods. Important components to our project are the studies pertaining to soil health in organic systems. Identifying soil microbial community profiles that are linked with disease suppression has also been recognized as a priority area of investigation by the USDA (Koenig, 2002). The dynamics of organic farming systems is an area of research that has been neglected in the past, however, it now represents an area of significant potential income in many southern states.
Hypothesis of the study
Considering scientific findings reviewed above and potential positive interactions of biofumigation and compost, we hypothesized that the combination of mustard meal applicaiton with compost might improve biological characteristics of the soil if compared with the incorporation of mustard meal along or chemical fumigants and stabilize these positive characteristics with time. The improvement of soil health will lead to a decrease of plant diseases and to an increase of the plant yield.
Objectives of the project were:
1) Test the effect of biofumigation (cover crop, different rates of mustard meal and compost application) on tomato yield, quality and disease development in the field experiments at different locations.
2) Determine long-term influence of mustard meal and compost treatments on strawberry yield, quality and disease in the field experiment.
3) Evaluate different techniques of MM application and timings of compost incorporation
4) Test the effect of biofumigation (cover crop, mustard meal and compost application) on bacterial and fungal characteristics of the soil and nematode population in the field experiments and commercial sites at different locations.
5) Test the effect of biofumigation (rates of MM and compost application) on weeds suppression.
6) Validate the optimal combinations of mustard meal and compost in experiments with commercial growers.
7) Develop a database of the collected information and present results of the project at the webpage “Enhanced Biofumigation & Composting”