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. Transplant growers were advised to apply a fixed copper bactericide beginning 2 ½ weeks after seeding at 5-day intervals for a total of 5 applications. For field growers, the recommendation was to start to apply the copper within 7 days after transplanting — applying at least 3 applications at 7-day intervals. 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.
In the decade or so since these protocols were developed 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 (Table 1). 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.
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 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.
Summary of Efficacy Trials
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.
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.