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Tomatoes rank second in terms of cash receipts among all vegetables produced in the United States, with a net value of more than $1.6 billion and a total production of 12 million tons (Swiader and Ware, 2002). Bacterial wilt caused by Ralstonia solanacearum is a serious disease and a major constraint in production of tomatoes. This disease causes rapid wilt and death of the tomato plant; and incidence can be as high as 50% or more in the southern US and subtropical and tropical areas of the world (Jones et al., 1991).

The efficacy of current strategies for control of bacterial wilt in tomatoes is limited. Conventional pesticides that are known to provide effective control of the disease are not available. As a soil-borne disease with systemic infection through the xylem, bacterial wilt cannot be controlled by foliar application of traditional pesticides such as copper and mancozeb, which are used for control of foliar bacterial diseases.

Soil treatments with general-purpose fumigants like methyl bromide did not provide satisfactory control of the disease (Chellemi et al., 1997; Enfinger et al., 1979). Control of bacterial wilt in infested soils is very difficult. It is generally considered that crop rotation with a non-host crop is of minimal value because of the wide range of crop and weed hosts of the pathogen (Hayward, 1991; Pradhanang and Momol, 2001). Once the pathogen becomes established in the field, previously productive tomato fields may have to be abandoned due to serious outbreak of the disease. The destructive nature of the disease and ineffective disease suppression of current control measures have made development of effective disease prevention approaches desirable.

The objective of this study was to evaluate the efficacy of several Improcrop compounds for control of bacterial wilt of tomatoes. The compounds were used for soil and seed treatments in an attempt to reduce pathogen populations combined with application of compounds that may induce resistance of tomato plants for an integrated management of this disease.


Materials and methods

BACTERIAL CULTURE AND INOCULUM PREPARATION

R. solanacearum tomato strain Rs5 (race 1, biovar 1) isolated in Quincy, Florida (Pradhanang and Momol, 2001) was used in this study. The bacterial pathogen was grown on casamino acid peptone glucose medium (CPG) (Kelman, 1954) at 28oC for 48 hrs. Bacterial cells were suspended in sterile deionized water and concentration of inoculum was estimated by measuring absorbance at 590 nm. The viable bacterial population was determined following dilution and plating on CPG agar. Soils used for tomato transplanting were infested with the pathogen at an initial density of 6 x 107 CFU/ml of soil.


APPLICATION OF IMPROCROP COMPOUNDS

Three compounds were used in the study, Stubble- AidTM, ISR 2000TM and AgromosTM. Tomato seeds of a susceptible variety (Solar Set) were treated with Stubble-AidTM solution (50%) for 30 min before seeding. The seeds were sown in Terra-lite® agricultural mix (Scott Sierra Horticultural Products Co., Marysville, OH) in expanded polystyrene flats with 2.5 cm by 2.5 cm cells and maintained in a greenhouse.

The compound ISR 2000TM was applied as a foliar spray and soil drench at a concentration of 1 ml/L starting 15 days before tomato seedling transplanting and once every seven days thereafter for two more applications. Soils (the potting mix) used for transplanting were either treated, or not treated, with Stubble-AidTM solutions at the rate of 1% two hours after soil infestation with the pathogen.

Tomato seedlings were transplanted into plastic ‘Cone-tainers’ (20.5 x 4.0 cm) containing the Stubble-AidTM-treated or not treated soils four days after soil treatment with Stubble-AidTM. AgromosTM was applied as a foliar spray at weekly intervals after transplanting at a concentration of 1.2 ml/L. Nu-Film 17 was used as adjuvant (0.125%) in AgromosTM solution. Untreated tomato plants growing in the infested soils were used as a control. A subset of tomato plants treated (as described above) or not treated with these compounds, was grown in soils not infested with the pathogen to evaluate the effect of these compounds on plant growth. The plants were supplied with Peter’s Peat Lite Special (15:16:17 NPK) at weekly intervals at the rate of 7.5 g /L water.


EXPERIMENT DESIGN AND DISEASE ANALYSIS

A randomized complete block design was employed with five replications and five plants for each treatment in one replication. Disease incidence was recorded weekly after tomato transplanting and quantified as the percentage of plants wilted. Plant height and the weight of roots and foliage were measured at the end of the experiment. The data were analyzed using the ANOVA or GLM procedures of the Statistical Analysis System (SAS Institute, Cary, NC). Means were compared using Duncan’s multiple range test at P = 0.05.


Results and discussion

Application of Stubble-AidTM in conjunction with ISR 2000TM and AgromosTM consistently provided significant protection of tomato plants against bacterial wilt. Ten days after inoculation the protection of tomato plants from R. solanacearum was 100% (no treated plants wilted). Twenty-one days after inoculation 24% of inoculated treated plants wilted (Figure 1). Disease incidence of treated plants was 40% four weeks after transplanting while 100% of untreated plants wilted (Table 1, Figures 2 and 3).

Application of these compounds also showed a significant enhancement of plant growth as indicated by larger plant and root weight (Table 2).

The results indicated that these compounds exhibited the potential to significantly reduce bacterial wilt incidence and enhance growth of the susceptible tomato cultivar. The mode of action(s) involved in the disease reduction by these compounds is unclear.

Treatments of the soil and tomato seeds with Stubble- AidTM might have reduced populations of R. solanacearum in the soil and prevented colonization of the root system by the pathogen. ISR 2000TM and AgromosTM may have the potential to enhance resistance of tomato plants against this pathogen and reduce disease incidence by induced resistance. It is worthwhile to carry out further experiments to elucidate the mechanisms associated with disease reduction by these compounds.

Detailed studies will also be necessary to investigate the timing and application dose needed to sustain the high efficacy that was observed during the early stage (10 days after inoculation). These studies will help optimize the application methodology of these compounds to facilitate their practical use for bacterial wilt management in tomato production.


Table 1. Effect of Stubble-AidTM and other compounds on disease incidence of bacterial wilt of tomato (cv. Solar Set) under greenhouse conditions.


^aStubble-AidTM was used to treat seeds and soils, ISR 2000TM was applied as foliar spray and soil drench, and AgromosTM was applied as foliar spray.
^bInoculation or no inoculation indicates seedlings were transplanted into soils infested or not infested with R. solanacearum.
^cValues are the means of five replications (25 plants). Same letter in each column indicates no significant difference based on Duncan’s
multiple range test at P = 0.05.
^dDays after inoculation.



Table 2. Effect of Stubble-AidTM and other compounds on the growth of tomato plants (cv. Solar Set) in bacterial wilt greenhouse trial.


^aStubble-AidTM was used to treat seeds and soils, ISR 2000TM was applied as foliar spray and soil drench, and AgromosTM was applied as foliar spray (see descriptions in the text for timing and dosage of applications).
^bInoculation or no inoculation indicates seedlings were transplanted into soils infested or not infested with R. solanacearum.
^cValues are the means of five replications (25 plants). Same letter in each column indicates no significant difference based on Duncan’s multiple range test at P = 0.05.
^dPlant growth was evaluated 36 days after inoculation.





Figure 1. All treatments: photo taken 21 days after inoculation or transplanting.





Figure 2. Uninoculated treatments: photo taken at the end of the experiment.





Figure 3. Inoculated treatments: photo taken at the end of the experiment.



References

Chellemi, D.O., S.M. Olson, D.J. Mitchell, I. Secker and R. McSorley. 1997. Adaptation of soil solarization to the integrated management of soilborne pests of tomato under humid conditions. J. Phytopathology 87:250-258.

Enfinger, J.M., S.M. McCarter and C.A. Jaworski. 1979. Evaluation of chemicals and application methods for control of bacterial wilt of tomato transplants. J. Phytopathology 69:637-640.

Hayward, A.C. 1991. Biology and epidemiology of bacterial wilt caused by Pseudomonas solanacearum. Annu. Rev. Phytopathol. 29:65-87.

Jones, J.B., J.P. Jones, R.E. Stall and T.A. Zitter. 1991. Compendium of tomato diseases. APS Press, St. Paul, MN.

Kelman, A. 1954. The relationship of pathogenicity in Pseudomonas solanacearum to colony appearance on a tetrazolium medium. J. Phytopathology 44:693-695.

Pradhanang, P.M. and M.T. Momol. 2001. Survival of Ralstonia solanacearum in soil under irrigated rice culture and aquatic weeds. J. Phytopathology 149:707-711.

Swiader, J.M. and G.W. Ware. 2002. Producing vegetable crops. Interstate Publishers, Inc., Danville, IL.

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