Managing Bacterial Spot in Ontario Tomatoes: Updated Best Management Practices for Field Production

Updated by: Cheryl Trueman, Ridgetown Campus, University of Guelph; Amanda Tracey, OMAFRA, Ridgetown; Tina Simonton, M.Sc. Candidate, Department of Plant Agriculture, University of Guelph

Original by: Cheryl Trueman, Ridgetown Campus – University of Guelph; Janice LeBoeuf, OMAFRA (former), 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. This strategy was mainly to suppress populations early in the season while they are still low and the potential impact on yield and quality is highest. However, given poor performance of this approach in years with favourable conditions, best management practices integrating multiple approaches for management in field production were introduced in 2015. Since 2016, we have worked on validating some of those suggested practices. We also continued evaluations of bactericides for management in field tomatoes. The following is a summary of what we found and an updated table of suggested best management practices for bacterial spot management in field tomatoes.

Efficacy Trials

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 eight applications of Kocide 2000 + Actigard beginning within seven days of transplanting, applied at 7-day intervals (Table 1). This treatment resulted in measurable disease reduction in all years, but it did not always increase yield or reduce spotting on fruit. Other copper-based programs, as well as other tested products, were inconsistent or ineffective in these trials, and in an additional trial completed in 2018. The efficacy data suggests that growers will not see an economic benefit from copper applications for bacterial spot management in field tomatoes.

These results are consistent with those from a survey of the Ontario processing tomato industry we 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 (AAFC) 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?

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. In 2016, we suggested a new focus on best management practices to exclude the pathogen from tomato cropping systems and reduce its spread using biosecurity and sanitation practices common in the greenhouse industry. The overarching strategy is to adopt multiple practices to limit spread and delay an epidemic of bacterial spot as much as possible. We evaluated the potential transmission of X. gardneri during transplant shipping and transplanting and assessed the efficacy of disinfectants on trailer and transplanter surfaces.

Transmission during transplant shipping

Using a mock trailer setup, the appearance of symptoms after irrigating a) top to bottom, b) bottom to top, and c) using a tray dip, was evaluated. A tray with symptomatic seedlings on the top shelf of each mock trailer served as a source of inoculum. X. gardneri moved downward from symptomatic seedlings to healthy seedlings in a simulated plug trailer setup after irrigation (Table 2). Disease incidence on seedlings irrigated with a spray nozzle inside the trailer was higher than a tray dip outside the trailer, but transmission still occurred with a tray dip (Table 3). The results show that X. gardneri can spread easily within plug trailers when seedlings are irrigated.

Transmission during transplanting

Indoors: X. gardneri was present on previously healthy tomato seedlings that passed through the transplanter after seedlings with bacterial spot symptoms (Table 4), showing contaminated transplanting equipment is a pathway for spread.

Outdoors: Transplants with bacterial spot symptoms were passed through a transplanter before healthy seedlings and bacterial symptom appearance was monitored. Symptoms usually appeared at the same time whether symptomatic seedlings were planted using a transplanter or manually after transplanting (Figure 1). This means that even though X. gardneri can be transmitted on transplanting equipment, weather conditions play a key role in dissemination and symptom appearance (otherwise, the pattern of appearance among the two inoculated treatments would be different). Strict measures to clean and sanitize equipment and worker hands during transplanting may not be worth it, but there could still be value in implementing these measures when moving between fields, once or twice a day in the same field, and when switching varieties or transplant supplier within a field.

Disinfecting Equipment and Structures

There are four major types of disinfectants: bleach, quaternary ammonium compounds (i.e. KleenGrow and Chemprocide), peroxygen compounds (i.e. hydrogen peroxide and Virkon) and alkalis (i.e. C-Clean and TSP). It is important to consider the material you are disinfecting when choosing a product.

To properly clean and disinfect there are six important steps:
1. Remove excess organic matter by brushing, spraying and/or rinsing.
2. Use a pre-disinfection washing agent or detergent and apply at a low pressure (50-100psi).
3. Repeat step one for particularly difficult surfaces (i.e. rubber and plastic) or surfaces that may have a high level of inoculum.
4. Rinse well with water to remove any detergent residues and organic matter that may interfere with the disinfectant.
5. Allow the surface to dry completely.
6. Apply the disinfectant at label rates and allow it to remain wet on the surface for 15-30 minutes.

Always read product labels and directions thoroughly to ensure the best product efficacy and user safety.

To evaluate the efficacy of common disinfectants to clean transplant trailer and transplanter surfaces, we completed two trials using Grade 319 caste aluminum and tarp (18 oz vinyl). Materials were dipped in a solution of X. gardneri, allowed to dry overnight, washed in soapy water (0.1% v/v Dawn dish soap), rinsed in tap water, dipped in the treatment solution for 2 minutes, allowed to sit for 10 min (bleach for 30 min), rinsed in tap water, and then air dried. Contact plates were applied to the surfaces, incubated for three days and the total bacterial count determined (Table 5). Overall, the most consistent treatment was washing with soap and water in combination with Hyperox, but Oxidate 2.0, Virkon Greenhouse, and Clorox were also effective in most trials. Kleengrow was not effective on tarp material and effective in only one of two trials on aluminum using this method.

Updated Best Management Practices for Bacterial Spot Management in Tomato

Emphasizing tactics that exclude the pathogen from tomato cropping systems and reduce spread within the crop is not a simple strategy, and there are costs involved, but keep in mind the cost of spray programs that have demonstrated little benefit in reducing bacterial spot severity. The strategies presented below may help reduce the impact of bacterial spot on field tomatoes in Ontario and are ranked by the level of expected impact.

Additional resources

Good Agricultural Practices: Cleaning and Sanitizing
Sanitation Guidelines for Management of Pest and Disease of Greenhouse Vegetables 
Biosecurity is Important in Horticulture Crops too (Nova Scotia)

Acknowledgements: This research was supported by the Ontario Agri-Food Innovation Alliance and the Ontario Tomato Research Institute.