Management of bacterial spot in field tomatoes: recent research results from Ridgetown Campus

Cheryl Trueman, Ridgetown Campus – University of Guelph; Janice LeBoeuf, OMAF & MRA, Ridgetown

Learn more. See updated management strategy for Ontario field tomatoes (March 26, 2015)

Bacterial spot is one of the most important production challenges for field tomatoes in Ontario. Bacterial spot is not caused by one pathogen, but by a number of separate species in the genus Xanthomonas. The predominant species of bacterial spot in Ontario has shifted in the last fifteen years or so, to one known as Group D or Xanthomonas gardneri.  This has proven to be a more aggressive pathogen than the Group A and B species that were formerly common in Ontario field tomatoes.

Although many products and management strategies have been tested over the years to try to control bacterial spot, the fixed copper bactericides have continued to be popular management tools. While there are now a few additional products registered for bacterial spot suppression in tomatoes in Canada, they should still be used in combination with a copper program.

Characteristics of copper as a bactericide

In order to use copper most effectively, it is important to understand its characteristics.  The copper bactericides are protectants, with no systemic activity.  They form a protective coating on the plant surfaces, but do not redistribute as the tissue expands.  When the plant surface is wet, the copper particles slowly dissolve, releasing ions that will negatively affect the bacterial cells that come into contact with them.  High solubility copper formulations increase the potential for plant damage and decrease residual activity, so fixed coppers are the better fungicides/bactericides. With fixed coppers, the copper ions are released slowly from the particles deposited on the plant.

Copper bactericides need to be reapplied often, due to the growth of the plant (new and expanding tissue is unprotected) and due to losses from rain and even wind.  Yes, wind can actually blow the larger copper particles off of the plant!  In wet, rainy conditions, and when growth is rapid, copper will have to be reapplied more often than in dry conditions.

When looking at formulations, there are many factors that influence efficacy such as:

• Particle size:  Smaller particle sizes have more surface area to release copper ions (the part that is active on the bacteria). Smaller particles are not as susceptible to loss through rain and wind. Note that particle size cannot be determined by looking at the product. The size of the granule in the formulated product is not related to individual particle size.
• Elemental (metallic) copper content:  Shown on the label.
• Surfactants in different formulations can affect rainfastness and spreading.

It is not possible to evaluate how well the product will perform based on these characteristics, however. During application and in the field there are other factors that come into play.  For this reason efficacy trials have been used to compare performance of various fixed copper products under field conditions in Ontario.

Effectiveness of copper and other products for bacterial spot management

Research trials to evaluate copper bactericides and other products for bacterial spot management in processing field tomatoes were completed at the Ridgetown Campus, University of Guelph from 2010 to 2013. Areas of focus included efficacy of different brands and formulations of copper, efficacy of copper alternatives, efficacy of copper products mixed with other products, and the influence of the number of applications of copper during the field season on bacterial spot epidemics. For all trials, at least one product application was made prior to inoculation with X. gardneri. In 2010-2012, trials were also inoculated with Pseudomonas syringae pv. tomato, which causes bacterial speck on tomatoes, but is considered less aggressive than X. gardneri.

Different copper formulations: The efficacy of different copper formulations was evaluated in 2010 (Table 1). Kocide 2000 (copper hydroxide 53.8%, Kocide 3000 (copper hydroxide 46.1%, Parasol WP (copper hydroxide 50%), Parasol FL (copper hydroxide 24.4%), Copper 53W (tri-basic copper sulphate 53%), and Copper Spray (copper oxychloride 50%) were applied at label rates using a 7-day interval, and began prior to pathogen inoculation. There was no difference in the level of defoliation among copper products on August 13, which was just prior to harvest (Figure 1). All the copper products reduced defoliation compared to the nontreated control.

Efficacy of products applied alone, or in tank-mixes: Several products have been evaluated at the Ridgetown Campus since 2010 for suppression of bacterial spot. Treatment applications began within seven days of transplanting and were repeated eight times. Table 1 summarizes treatments that reduced disease severity early in the season (ie. up to early to mid-July), as measured by the number of leaves with symptoms, compared to the nontreated control. Kocide 2000 applied alone, or with Dithane, was effective at suppressing bacterial disease in 2010 and 2011, but not in 2012 and 2013. Applications of Kocide 2000 combined with Actigard have resulted in the most consistent suppression. However, there are no ‘silver bullet’ solutions to bacterial spot management. There are few cases where early season disease suppression results in significant reduction in defoliation compared to the control later on in the season (Figure 2). This emphasizes the need to limit introduction of X. gardneri into a tomato crop as long as possible.

Effect of the number of field applications of copper on disease epidemics: To determine whether or not there is an advantage to repeated applications of copper during the growing season, the effect of number of copper applications on disease severity was also evaluated in 2012 and 2013. Three, six, nine, or 11 applications were made, which corresponded to vegetative growth, fruit set, fruit ripening, and 14 days before harvest. All programs resulted lower disease severity than the nontreated control (0 applications) during the period up to mid-July in 2012, but had no effect on disease incidence in 2013 (Figure 3). 2012 was a very dry and hot year, compared to 2013 which had periods of heavy rainfall in June and early July, and more moderate temperatures. There was no effect on defoliation later on in the season in either year.

Conclusions

Early season applications of copper and some other products can suppress establishment of bacterial spot, but results are not always consistent from year to year. However, suppression with these treatments will not always be detected at the end of the season, and there is little evidence so far to suggest that season-long applications of copper will provide season-long suppression of bacterial spot. Under conditions of heavy rains and stormy conditions that wound leaves, it is unlikely that any products will provide good control of bacterial spot in the field.

Keep in mind the following tips when making applications of copper and other products to tomatoes in the field:

• You will not be able to ‘rescue’ a field with applications of a bactericide; if bacterial spot is well established continued applications are unlikely to help.
• Apply PREVENTATIVELY. Protect new crop as early as possible.
• Apply at recommended intervals; protect new growth as all copper products are CONTACT products.
• Apply under conditions to obtain maximum COVERAGE.

Starting the season with healthy, pathogen-free transplants is still critical to prevent and delay establishment of bacterial spot in the field. Current research at the Ridgetown Campus is focussing on evaluating seed and foliar treatments to protect tomato seedlings from X. gardneri.

Acknowledgements: Portions of this research were funded by the Ontario Tomato Research Institute, OMAF & MRA – U of G Partnership, and Syngenta Canada.