Stephen M. Olson

Management of Tomato Bacterial Spot in the Field by Foliar Applications of Bacteriophages and SAR Inducers

Various combinations of the harpin protein, acibenzolar-S-methyl, and bacteriophages were compared for controlling tomato bacterial spot in field experiments. Harpin protein and aciben-zolar-S-methyl were applied every 14 days beginning twice before transplanting and then an additional four applications throughout the season. Formulated bacteriophages were applied prior to inoculation followed by twice a week at dusk. A standard bactericide treatment, consisting of copper hydroxide plus mancozeb, was applied once prior to inoculation and then every 7 days, while untreated plants served as an untreated control. Experiments were conducted in north and central Florida fields during fall 2001, spring 2002, and fall 2002. In three consecutive seasons, acibenzolar-S-methyl applied in combination with bacteriophage or bacteriophage and harpin significantly reduced bacterial spot compared with the other treatments. However, it did not significantly affect the total yield compared with the standard or untreated control. Application of host-specific bacteriophages was effective against the bacterial spot pathogen in all three experiments, providing better disease control than copper-mancozeb or untreated control. When results of the disease severity assessments or harvested yield from the bacteriophage-treated plots were grouped and compared with the results of the corresponding nonbacteriophage group, the former provided significantly better disease control and yield of total marketable fruit.

Evaluation of thymol as biofumigant for control of bacterial wilt of tomato under field conditions

Volatile plant essential oils thymol and palmarosa oil, used at a concentration of 0.7%, were evaluated under field conditions for control of bacterial wilt of tomato caused by Ralstonia solanacearum. The experimental fields were artificially infested with the bacterial pathogen. Two hours after infestation, the plant essential oils were applied, then the plots were sealed with plastic mulch for 3 or 6 days. Tomato seedlings were transplanted into the field 7 days later. In fall of 2002, 92.5% of tomato plants (cv. Equinox) wilted in the untreated control plots. Both thymol and palmarosa oil soil treatments reduced bacterial wilt incidence significantly. Thymol was more effective than palmarosa oil based on the final assessment, when 33.1 and 48.1% of the plants had wilted in plots treated with thymol and palmarosa oil, respectively. Soil treatment with either thymol or palmarosa oil produced significantly higher yield of tomato than the untreated control. In 2003, only thymol was evaluated. Thymol application significantly reduced bacterial wilt incidence on the susceptible cultivar Solar Set. Disease incidence in untreated plots reached 65.5%, while in thymol treated plots only 12% of plants wilted. Thymol treatment also increased yield of Solar Set significantly compared with the untreated control. This is the first report on the use of thymol for controlling a plant disease under field conditions, which indicated that this compound provided effective control of bacterial wilt on susceptible tomato cultivars when used as preplant treatment of soils. Because of its volatile property and broad-spectrum functions, thymol shows potential to be used as a soil biofumigant for the management of various plant pathogens.

Improved efficacy of newly formulated bacteriophages for management of bacterial spot on tomato

Bacteriophages are currently used as an alternative method for controlling bacterial spot disease on tomato incited by Xanthomonas campestris pv. vesicatoria. However, the efficacy of phage is greatly reduced due to its short residual activity on plant foliage. Three formulations that significantly increased phage longevity on the plant surface were tested in field and greenhouse trials: (i) PCF, 0.5% pregelatinized corn flour (PCF) + 0.5% sucrose; (ii) Casecrete, 0.5% Casecrete NH-400 + 0.5% sucrose + 0.25% PCF; and (iii) skim milk, 0.75% powdered skim milk + 0.5% sucrose. In greenhouse experiments, the nonformulated, PCF-, Casecrete-, and skim milk-formulated phage mixtures reduced disease severity on plants compared with the control by 1, 30, 51, and 62%, respectively. In three consecutive field trials, nonformulated phage caused 15, 20, and 9% reduction in disease on treated plants compared with untreated control plants, whereas plants treated with PCF- and Casecrete-formulated phage had 27, 32, and 12% and 30, 43, and 24% disease reduction, respectively. Plants receiving copper-mancozeb treatments were included in two field trials and had a 20% decrease in disease in the first trial and a 13% increase in the second one. Skim milk-formulated phage was tested only once and caused an 18% disease reduction. PCF-formulated phage was more effective when applied in the evening than in the morning, reducing disease on plants by 27 and 13%, respectively. The Casecrete-formulated phage populations were over 1,000-fold higher than the nonformulated phage populations 36 h after phage application.

Integration of Biological Control Agents and Systemic Acquired Resistance Inducers Against Bacterial Spot on Tomato

Two strains of plant growth-promoting rhizobacteria, two systemic acquired resistance inducers (harpin and acibenzolar-S-methyl), host-specific unformulated bacteriophages, and two antagonistic bacteria were evaluated for control of tomato bacterial spot incited by Xanthomonas campestris pv. vesicatoria in greenhouse experiments. Untreated plants and plants treated with copper hydroxide were used as controls. The plant growth-promoting rhizobacteria or a tap water control were applied as a drench to the potting mix containing the seedlings, while the other treatments were applied to the foliage using a handheld sprayer. The plant growth-promoting rhizobacteria strains, when applied alone or in combination with other treatments, had no significant effect on bacterial spot intensity. Messenger and the antagonistic bacterial strains, when applied alone, had negligible effects on disease intensity. Unformulated phage or copper bactericide applications were inconsistent in performance under greenhouse conditions against bacterial spot. Although acibenzolar-S-methyl completely prevented occurrence of typical symptoms of the disease, necrotic spots typical of a hypersensitive reaction (HR) were observed on plants treated with acibenzolar-S-methyl alone. Electrolyte leakage and population dynamics experiments confirmed that acibenzolar-S-methyl-treated plants responded to inoculation by eliciting an HR. Application of bacteriophages in combination with acibenzolar-S-methyl suppressed a visible HR and provided excellent disease control. Although we were unable to quantify populations of the bacterium on the leaf surface, indirectly we determined that bacteriophages specific to the target bacterium reduced populations of a tomato race 3 strain of the pathogen on the leaf surface of acibenzolar-S-methyl-treated plants to levels that did not induce a visible HR. Integrated use of acibenzolar-S-methyl and phages may complement each other as an alternative management strategy against bacterial spot on tomato.

Application of Acibenzolar-S-Methyl Enhances Host Resistance in Tomato Against Ralstonia solanacearum

The chemical elicitor acibenzolar-S-methyl (ASM; Actigard 50 WG), which induces systemic acquired resistance (SAR), was investigated to determine the effect on bacterial wilt of tomato caused by Ralstonia solanacearum on moderately resistant cultivars under greenhouse and field conditions. In greenhouse experiments, ASM was applied as foliar spray and/or soil drench (3 μg/ml) before and as foliar spray (30 μg/ml) after transplanting. The chemical elicitor was ineffective in reducing bacterial wilt incidence on susceptible tomato cultivars Equinox and FL 47 when plants were inoculated with R. solanacearum. However, greenhouse studies indicated that ASM significantly enhanced resistance in cultivars with moderate resistance to bacterial wilt such as Neptune and BHN 466. It appeared that ASM-mediated resistance was partially due to prevention of internal spread of R. solanacearum toward upper stem tissues of tomato plants. The effect of ASM on moderately resistant cultivars was consistent in field experiments conducted in 2002 and 2003 in Quincy, FL, where bacterial wilt incidence was significantly reduced in ASM-treated BHN 466 (in 2002), FL 7514 (in 2003), and Neptune (both years) plants. ASM-treated BHN 466 and FL 7514 produced significantly higher tomato yield than the untreated controls. This is the first report of ASM-mediated control of bacterial wilt under field conditions, which suggests that use of this treatment for moderately resistant genotypes may be effective for control of bacterial wilt of tomato.

Photocatalysis: Effect of Light-Activated Nanoscale Formulations of TiO2 on Xanthomonas perforans and Control of Bacterial Spot of Tomato

Protection of crops from bacterial diseases presents a continuing challenge, mandating the development of novel agents and approaches. Photocatalysis is a process where chemically reactive oxygen species are catalytically generated by certain minerals in the presence of light. These reactive oxygen species have the capacity to destroy organic molecular structures critical to pathogen viability. In this study, the antibacterial potential of photocatalytic nanoscale titanium dioxide (TiO(2)), nanoscale TiO(2) doped (incorporation of other materials into the structure of TiO(2)) with silver (TiO(2)/Ag), and nanoscale TiO(2) doped with zinc (TiO(2)/Zn; AgriTitan) was evaluated against Xanthomonas perforans, the causal agent for bacterial spot disease of tomato. In vitro experiments on photocatalytic activity and dose dependency were conducted on glass cover slips coated with the nanoscale formulations by adding a known population of X. perforans strain Xp-F7 and illuminating the cover slips under a visible light source. TiO(2)/Ag and TiO(2)/Zn had high photocatalytic activity against X. perforans within 10 min of exposure to 3 × 10(4) lux. Greenhouse studies on naturally and artificially infected transplants treated with TiO(2)/Zn at ≈500 to 800 ppm significantly reduced bacterial spot severity compared with untreated and copper control. Protection was similar to the grower standard, copper + mancozeb. The use of TiO(2)/Zn at ≈500 to 800 ppm significantly reduced disease incidence in three of the four trials compared with untreated and copper control, and was comparable to or better than the grower standard. The treatments did not cause any adverse effects on tomato yield in any of the field trials.

Development of an Integrated Approach for Managing Bacterial Wilt and Root-Knot on Tomato Under Field Conditions

A 2-year field study was conducted to develop a field application method using thymol as a preplant soil treatment for controlling bacterial wilt and root-knot nematode on tomato (Lycopersicon esculentum). In addition, acibenzolar-S-methyl (ASM), which induces plant systemic resistance, was applied in conjunction with thymol to determine whether combining these tactics could improve bacterial wilt management. The test sites were artificially infested with Ralstonia solanacearum and Meloidogyne arenaria, and thymol was applied as preplant fumigation through drip irrigation lines under polyethylene mulch at a rate of 73 kg/ha in both 2004 and 2005. ASM was applied primarily as foliar spray at a concentration of 25 mg/liter. Application of thymol significantly reduced incidence of bacterial wilt on tomato in both years of the trial. In thymol-treated plots, 26.0 and 22.6% of the plants wilted in 2004 and 2005, respectively; whereas, in untreated plots, more than 95% of the plants wilted in each year. Number of root-knot nematode juveniles was significantly reduced in field plots treated with thymol and ASM for both years. The combined use of thymol and ASM provided the greatest reduction of root galling among the treatments. Tomato yield (cv. FL47) was evaluated only in the 2005 trial; thymol-treated plots produced significantly higher marketable yield than untreated plots, and the thymol treatment in combination with ASM significantly increased tomato yield compared with thymol or ASM alone. These results indicate that use of thymol and ASM was beneficial in controlling bacterial wilt and root-knot. We developed an effective method for applying thymol through drip irrigation lines for managing these diseases in tomato production.

Suppression of the Bacterial Spot Pathogen Xanthomonas euvesicatoria on Tomato Leaves by an Attenuated Mutant of Xanthomonas perforans

ABSTRACT A bacteriocin-producing strain of the bacterial spot of tomato plant pathogen, Xanthomonas perforans, with attenuated pathogenicity was deployed for biocontrol of a bacteriocin-sensitive strain of the genetically closely related bacterial spot of tomato plant pathogen, X. euvesicatoria. The attenuated mutant (91-118ΔopgHΔbcnB) of X. perforans was tested in leaf tissue and shown to significantly inhibit internal populations of the wild-type X. euvesicatoria strain although significantly less than the wild-type 91-118 strain, whereas in a phyllosphere inhibition assay, the mutant strain reduced epiphytic populations comparably to 91-118. Thus, the attenuated mutant limited the sensitive bacterium more efficiently on the leaf surface than inside the leaf. In field experiments, weekly application of 91-118ΔopgHΔbcnB significantly reduced X. euvesicatoria populations compared to the growers’ standard control (copper hydroxide and mancozeb applied weekly and acibenzolar-S-methyl applied every 2 weeks). The biological control agent, 91-118ΔopgHΔbcnB, applied every 2 weeks also significantly reduced X. euvesicatoria populations in one season but was not significantly different from the growers’ standard control. Potentially, attenuated pathogenic strains could be deployed as biological control agents in order to improve disease control of foliar plant pathogens.

Detection of Ralstonia solanacearum in Irrigation Ponds and Aquatic Weeds Associated with the Ponds in North Florida

The discovery of exotic Ralstonia solanacearum biovar 1 strains on geranium in north Florida led to a concern that this strain may have become established. Therefore, we monitored irrigation ponds and potential alternative aquatic weeds from 2002 to 2005 for the presence of this strain. We report that this strain, possibly originating from the Caribbean, has become established in several ponds in Gadsden County, FL. Cladistic taxonomy was used to subclassify the bacterium at the species level into four groups or phylotypes based on multiplex polymerase chain reaction of the internal transcribed spacer (ITS) region. The bacterium was further divided into sequevars by sequencing the endoglucanase gene (egl). The strains were determined to belong to phylotype II/sequevar 4 NPB (nonpathogenic on banana) that was recently reported in Martinique. Partial sequencing of the egl followed by phylogenetic analysis placed the new Caribbean strains in a different clade than the typical Florida endemic strains. Pulsed-field gel electrophoresis (PFGE) revealed different haplotypes upon comparison of the collected pond strains and the Floridian strains. Based on PFGE polymorphism, egl sequencing, and phylogenetic analysis, the Caribbean strains were shown to be identical to the strain isolated from infected geranium plants. Experiments were undertaken to monitor R. solanacearum in irrigation ponds and associated weeds. R. solanacearum was detected in surface-disinfested common aquatic weeds growing in the irrigation ponds, including Hydrocotyle ranunculoides (dollar weed) and Polygonum pennsylvanicum (Pennsylvania smart weed). Both weeds were latently infected and showed no signs of wilt when collected. Two different Hydrocotyle spp. were artificially inoculated with R. solanacearum under greenhouse conditions and both developed symptoms 14 days post inoculation (dpi) and the bacterium was recovered from the tissues 42 dpi. There was a positive correlation between ambient temperature and R. solanacearum populations in irrigation water, as previously shown by other researchers.

Characteristics of Watermelon Pollenizer Cultivars for Use in Triploid Production

Abstract Due to the increasing popularity of seedless watermelons [Citrullus lanatus(Thumb.) Matsum. & Nakai] and decreasing sales of seeded watermelons, much of the U.S. production has shifted from seeded to seedless watermelons. To achieve successful fruit set in trip-loid watermelon, a diploid watermelon cultivar must be planted as a pollen source. The characteristics of four diploid cultivars were compared with determine how they may perform as pollenizers. Numbers of male flowers produced, degree of fruit set, and fruit weight were compared. All cultivars began flowering over a period of 7 days and all were still flowering when sampling was terminated at 64 days after field transplanting. The cv. ‘SP-1’ consistently had higher numbers of male flowers than did cvs. ‘Companion’, ‘Jenny’, or ‘Mickylee’ through most of the season. ‘SP-1’ set the most fruit while ‘Companion’ set the least. ‘Mickylee’ had the largest melons and ‘SP-1’ the smallest. All cultivars were in peak male flower production during triploid fruit set and all would be expected to produce adequate seedless watermelon yields.