Managing Bacterial Spot in Ontario Field Tomato Production

Cheryl Trueman, Ridgetown Campus – University of Guelph; Janice LeBoeuf, OMAFRA, Ridgetown

Bacterial spot, caused by a group of Xanthomonas bacteria, is an ongoing challenge for field tomato growers in Ontario. For many years, a program of fixed copper sprays was used to manage bacterial spot in plug transplants and field tomatoes. Knowing that copper and other products are relatively weak on bacterial disease, the strategy was to suppress populations early in the season while they are still low. Once symptoms are present, the bacterial populations are so high that we would not expect to have a significant impact on disease development with a spray program.

Efficacy Trials

In recent years there have been several new products registered that include bacterial spot on their label, and many more efficacy trials to evaluate the field performance of these treatments. In trials completed from 2010-2014 at Ridgetown Campus, University of Guelph, with a copper sensitive Xanthomonas gardneri isolate, the only consistent spray program year after year was 8 applications of Kocide 2000 + Actigard beginning within 7 days of transplanting, applied at 7-day intervals. This treatment resulted in measurable disease reductions in all years, although it did not always increase yield or reduce spotting on fruit (Syngenta has discontinued selling Actigard in Canada, but as of the date of this post, there are still supplies of the product available from many ag outlets in southern Ontario).

Other copper-based programs, as well as other tested products, were inconsistent or ineffective. The efficacy data suggests that growers will not see an economic benefit from copper applications for bacterial spot management in field tomatoes. The efficacy of copper and other treatments on tomato transplants continues to be evaluated at Ridgetown Campus.

These results are consistent with those from a survey of the Ontario processing tomato industry completed in 2014. Over 80% of the growers that responded had used a copper-based spray program in 2014, but only 18% of them thought it had helped to reduce losses to bacterial disease. Furthermore, Dr. Pervaiz Abbasi (Agriculture and AgriFood Canada) reports that more than 70% of bacterial spot causing Xanthomonas spp. isolated from tomato in southern Ontario in 2012 were resistant to copper.

New Strategy

What is clear is that if we hope to improve management of bacterial spot, we have to move beyond a spray program that has little or no effect on reducing losses in yield and quality. We suggest a new focus on tactics to exclude the pathogen from tomato cropping systems and reduce its spread. The overarching strategy is to adopt multiple practices to limit spread and delay an epidemic of bacterial spot as much as possible. We have developed a list of best management practices for field tomato growers in Ontario. Over the next three years, research will be completed at the Ridgetown Campus to validate many of these practices with funding from the Ontario Tomato Research Institute and the OMAFRA/University of Guelph Partnership program.

Tier 1 – likely to have the biggest impact
Shipping/ Picking Up Transplants	One crop per load. Do not arrive to pick up transplants with a trailer already containing host plants (tomatoes, pepper) from another greenhouse.
	Clean and sanitize plant trailer between loads. Use appropriate sanitation chemicals & concentrations (see Additional Resources).
Holding Transplants	Plants need ventilation. If holding plants overnight or for an extended period of time, provide adequate ventilation to the plant trailer, to avoid a build-up of humidity and condensation in the trailer, which would promote the growth of bacterial diseases.
Transplanting	Design plug watering system to minimize dripping or splashing onto other trays of plants. Bacterial pathogens can move in water.
	Clean and sanitize the transplanter (surfaces that contact plants and trays) between field and varieties. Use appropriate sanitation chemicals and concentrations.
	Transplanting crew cleans and sanitizes their hands or changes to new disposable gloves at every break. Bacteria can be spread from plant to plant on workers’ hands. Consider this: on a 6-row transplanter, covering 1.5 ac/hour with 13,000 plugs per acre, each worker touches 3,250 plugs per hour.
In-season	Manage irrigation to minimize wounding and duration of leaf wetness. If overhead irrigating, use low pressure systems (boom, not gun) to minimize wounding and splashing. If irrigating, aim to minimize the duration that leaves are wet (timing, air movement, weed control).
Tier 2 – some impact expected
Pre-season	Consider adding windbreaks to slow wind and wind-blown rain. Strong winds can open microscopic wounds on the tomato foliage — an entry point for bacterial pathogens. Bacterial pathogens can be transferred from plant to plant in wind-blown mists and droplets.
In-season	Avoid planting tomatoes immediately adjacent to other host crops (peppers, other tomatoes).
	Clean and sanitize sprayer / cultivator equipment between fields. Bacteria can be spread from field to field on equipment that comes into contact with the crop.
	In processing and unstaked fresh market tomatoes, eliminate hoeing beyond 3 or 4 weeks after transplanting. This will reduce leaf tearing once the rows start to fill in.
	Eliminate inter-row cultivating beyond 3 or 4 weeks after transplanting. This will reduce leaf tearing once the rows start to fill in.
	When working with staked plants (pruning, tying), clean and sanitize tools between each plant. Change gloves or clean and sanitize hands every row. Bacteria can be spread from plant to plant on tools and workers’ hands.
	Crop scouts and other visitors instructed to clean and sanitize hands or wear gloves prior to entering each field. Wearing plastic booties which are changed after each field will also limit the spread of soilborne pathogens from field to field.
	Use 8 applications of copper + Actigard, applied on a 7-day interval, starting within 7 days of transplanting. Five years of research trials at Ridgetown Campus have shown that this is the most consistent program for reducing early season disease and in some cases, reducing defoliation. It is the only program that has shown a yield benefit (in 1 year out of 5) in the research trials. In the absence of Actigard, no program exists which has shown effectiveness in Ridgetown research trials with 8 applications on 7-day intervals. Note: Syngenta Canada reports that supplies of Actigard are still available at many ag outlets in southern Ontario as of the date of this post.
Tier 3 – little impact expected compared to tiers 1 or 2
In-season	Controlling weeds in the field. Weeds are potential hosts for bacterial spot and interfere with air movement and drying of the crop canopy.
	Fallowing weak areas within the field (historic poor drainage, low areas). This may be where the severe symptoms show up first, but is probably not the initial source of the inoculum.
Tier 4 – no impact expected on bacterial spot
Pre-season	Crop rotation. Although beneficial for managing many crop pests, it is not as useful for managing bacterial spot.
In-season	Applying other protective spray programs, except the program listed in Tier 2. Five years of bacterial spot efficacy research at Ridgetown Campus has shown minimal to no benefit to any of the spray programs tested, except 8 applications of copper + Actigard as described in Tier 2.
	DSV-based spraying of bactericides. TomCAST is based on the biology of the fungal pathogens that cause early blight, septoria leaf spot, and anthracnose. DSV-based spray timing is not appropriate for bacterial spot management.
	Using disinfectant on tools, equipment, hands, and other surfaces without pre-cleaning to remove films and organic matter. Disinfectants must be applied to clean surfaces to be effective.
	Beginning a program of cleaning and disinfecting tools, equipment, hands, and other surfaces after disease is already established. Bacteria are present (and can be spread) long before the first symptoms are visible.

Do More Than Think About It

When adopting a new bacterial disease management program on-farm, it is critical to have it written down, to train the people who will be doing the work, and to keep records. Written protocols and/or checklists don’t have to be elaborate, but are needed to ensure activities are not forgotten. Review them often as a team; update them and make changes based on experience and new information.

In conclusion, the research shows that it is time to move beyond the spraying strategy and put the focus on other management practices. The emphasis must be on tactics to exclude the pathogen from tomato cropping systems and reduce its spread within the crop. This will require adoption of multiple practices to limit spread and delay an epidemic of bacterial spot as much as possible. This is not a simple strategy, and there are costs involved, but keep in mind the cost of spray programs that have marginal, if any, benefit and the cost in potential losses to bacterial spot.

Additional resources

On-Farm and Greenhouse Sanitation and Disinfection Practices to Minimize the Impact of Plant Pests (BC Ministry of Agriculture)