During the growing seasons of 2001 and 2002 field studies involving sustainable pre-plant treatments for use in plasticulture tomato production were trialed. Compost based treatments were found to be effective at both increasing yields and decreasing the incidence of Southern Blight. Biofumigation treatments also favorably influenced crop production although they were not as effective as synthetic fumigants. Low-dose chemical fumigation, when combined with organic amendments, proved to be a feasible alternative to full dose fumigation. Solarization treatments were implemented during the spring and did not significantly suppress soilborne diseases.
The use of raised-bed plasticulture systems 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). This production method is commonly used for field tomatoes (Lycopersicon esculentum Mill.) – an economically important agricultural industry in many states. Tennessee is the fourth largest tomato producer in the United States (based on 5,700 production acres reported by Rutledge, 1998; USDA, 1998). Tomato production in surrounding southeastern states represents almost 20,000 production acres.
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 in the southeast United States. This pathogen is also a hazard to many other agricultural investments throughout the U.S. making the investigation of pre-plant soil treatments for plasticulture tomato production under pressure from Southern Blight an important area of research.
Methyl bromide fumigation has routinely been heavily relied upon for pathogen management in modern high production raised-bed agriculture. There is an accumulation of experimental data supporting the concept that many non-chemically based pest management strategies are effective in controlling diseases caused by soilborne pathogens. The mode of action of many of these treatments has also been described. Composted amendments can introduce a varied microflora into soil ecology, including many biocontrol agents, while providing a suitable substrate to support their growth (Hoitink, 1994). Macerated tissues from Brassica cover crops are known to release biocidal compounds that are able to suppress soilborne pathogens when incorporated into the soil (Kirkegaard, 1998). Solarization has also been shown to reduce pathogen numbers by elevating soil temperatures to levels that deactivate their propagules (Katan, 1991). Sustainable control of soilborne phytopathogenic fungi is likely to be achieved through the enhancement of alternate control methods via the integration of multiple techniques (Cook, 1990). Sub-lethal doses of chemical fumigants 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).
The SARE-funded graduate research project based at The University of Tennessee investigated the methods required to successfully implement the various sustainable management techniques described above.
1. Determine the most effective biofumigation practices for controlling Southern Blight in tomato.
2. Determine the optimal composting system for enhancing yields and controlling soilborne pathogens.
3. Examine the efficacy of combining chemical fumigants with organic amendments.
4. Develop an integrated system of sustainable disease management that combines biofumigation, composting, and solarization in addition to, or exclusive of, synthetic chemical fumigants.
5. Prepare the model system for implementation in commercial tomato production.
The 2001 and 2002 field experiments were based on a split plot arrangement of a randomized complete block design with pH whole plot treatments and the pre-plant amendments as the subplot treatments. The field measured 250 x 100 ft. (76.3 m x 30.5 m) in total. It was divided into thirty rows measuring 100 x 4 ft. with 5 ft. centers between the rows and 12 ft. buffer zones between the three sets of 10 rows. Four blocks were designated running north to south, thus, 120 separate replicates of 25 x 4 ft. existed. Within each block, three whole plots were established, each 84 x 25 feet and containing 10 rows. Within each whole plot, the 10 subplots were available for individual treatment assignment. The field was limed to maintain whole plot treatments of soil pH in the range of 5.5-6.5, 6.5 -7.0 and >7.0. When formed into plastic mulched raised beds, each replicate measured 24 ft. x 28 in. with a two ft. buffer between each adjacent replicate.
The pre-plant treatments were as follows: 2001 – Biofumigation (Fall incorporation), Biofumigation (Spring incorporation), Biofumigation + compost, Compost, Compost + Dazomet@50% rate, Dazomet, Dazomet + compost, Solarization, Solarization + compost, Control. In 2002 Biofumigation + Dazomet@50% rate was substituted for the Biofumigation (Spring incorp.).
Commonly implemented production methods for plasticulture tomatoes were adhered to. In the fall of the year prior to each growing season 60 lb/acre of 10-10-10 was broadcast onto the field, as was 1 ton/acre of lime for pH maintenance. The beds were formed, black plastic laid, and trickle tape inserted in a one-pass operation. The tomato cultivar “Celebrity” was used. The transplants were planted through the plastic at 18 in. intervals. Twelve seedlings per 24 ft. bed were planted with the middle ten plants used for the experimental data. For one week following the original planting date any dead transplants were replaced as needed. After one pruning the plants were staked according to a Florida weave system. During the growing season the plants received trickle irrigation to equal 1”/week. Fertigation was applied through the drip irrigation system. After initial flower set, CaNO3 and KNO3 were alternated weekly @ 5lbs N/acre. Harvesting was conducted twice weekly with each replicate being picked into a separate, labeled bucket. Grading was performed using a mechanical grading table. Harvest weights were firstly recorded to a worksheet and then transferred to a computer spreadsheet. The incidence of Southern Blight was recorded throughout the season.
In 2001 the pre-plant treatments that had the greatest influence on yield were those based on a compost application. The treatments included compost alone, compost + solarization, and compost + 50% dazomet. The compost alone treatment recorded a mean yield of 7.45 kg of fruit per plant (Figure 1). This represented a 91% increase above the control plants. The biofumigation, solarization and dazomet treatments did not produce yields statistically significantly different from the controls.
Figure 3 displays the incidence of Southern Blight recorded in the experimental plots in 2001. The control plots developed 36 diseased plants out of 120 while the compost treatment and dazomet treatment had 8 and 5 diseased plants respectively. The compost + 50% dazomet treatment displayed the greatest ability for disease suppression, with only 1 diseased plant throughout the season. The biofumigation and solarization treatments were not able to reduce the incidence of Southern Blight to a statistically significant degree.
In the second year of the study, 2002, the separation between the experimental treatments and the controls became more pronounced pertaining to both yields and disease. The compost based treatments continued to produce the largest fruit yields. The compost, dazomet + compost, and compost + 50% dazomet treatments all recorded yields significantly different from the controls, but not significantly different from each other (Figure 2). In 2002 the biofumigation treatments also produced yields significantly higher than the controls.
The number of plants in the control beds that became infested with Sclerotium rolfsii, the causal agent of Southern Blight, increased to 89 in 2002 (Figure 4). The incidence of disease increased in all of the experimental treatments, however, several of the treatments were able to suppress the disease significantly better than the controls. The compost + 50% dazomet treatment was again the most effective, recording 19 diseased plants. All three of the biofumigation treatments were able to keep the number of diseased plants to approximately 50% of the controls.
Educational & Outreach Activities
IN PROGRESS: Dissertation – The University of Tennessee, Department of Plant and Soil Sciences: “Suppression of Soilborne Phytopathogenic Fungi of Tomatoes Utilizing Integrated Production Systems”, Martin Lyons.
Well-documented deleterious environmental consequences associated with the unmitigated use of pesticides are important reasons why fruit and vegetables growers should be looking toward integrated pest management as an alternative method of agricultural production. Soil treatments such as composting, biofumigation, and solarization act to establish and maintain a disease suppressive soil microecology. Such an environment will resist the introduction and proliferation of soilborne pathogens and reduce pesticide requirements.
The use of raised-bed plasticulture methods for fruit and vegetable production is becoming increasingly prevalent within the southern 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. Hence, research focusing on disease prevention methods applicable to this commonly used method of agricultural production is essential.
Fields can develop high pathogen inoculum densities when grown without rotation. Southern Blight, caused by the phytopathogenic fungus Sclerotium rolfsii, represents one of the major disease threats to tomato crops in the southeast United States. Vegetable crops grown in regions that have predominantly warm, humid growing seasons are susceptible to this disease. Significant economic losses have been attributed to Southern Blight in tomato, green beans, cantaloupe, peppers, and potatoes. Although difficult to assess, the economic losses caused by S. rolfsii can be quite severe due to the possibility of total destruction of the crop in some species. Estimates have been made that at least 5% of the annual loss of crops in the southern United States could be attributed to this disease-causing organism.
Areas needing additional study
A significant quantity of research hours has been invested into assessing several sustainable pre-plant treatments for vegetable production in this research project. The effectiveness of these treatments to both enhance yields and suppress disease has been investigated throughout this multiple year study. Additionally, the mechanisms of the amendments and their impact on the soil microecology have been examined. Large-scale commercial application of the most successful treatments would be an advantageous progression. Refinement of the techniques involved for profitable commercial adoption of the system could be conducted on several regional farms. More study would be required to amalgamate the requirements for the successful implementation of the treatments within the parameters of a working vegetable production field.
Associated with the incorporation of biologically based treatments into agricultural production is the opportunity to investigate the microbiology that drives such pest management techniques. The growth and survival of numerous biological control agents, such as Trichoderma spp. and Bacillus spp., is of vital importance in the science of sustainable agriculture. Knowledge on the mechanisms involved with biocontrol and the suppression of soilborne diseases will relate directly to their successful utilization.
Closely linked to the farmer’s willingness to implement sustainable and organic agricultural techniques into their production practices is the understanding of new methods. Visual demonstrations, coupled with explanations of the procedures involved, would be of great value to the widespread implementation of the production system developed during this project. This could be accomplished at field days and via on-farm demonstrations.