New (and Old) Options for Management of Bacterial Spot and Speck in Field Tomatoes

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

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

Diseases such as bacterial spot and bacterial speck have been a major challenge in Ontario field tomato production for many years.  Management practices aimed at minimizing losses to bacterial disease begin in the greenhouse and continue through most of the season.

To review the greenhouse recommendation for tomato transplant production, a registered fixed copper bacteriacide should be applied according to label instructions, starting 2 ½ weeks after seeding, then every 5 days for a total of 5 applications.  Apply in sufficient water to wet foliage just to runoff, not to drench the plug.  This should be applied after the last watering of the day.

Field growers should ensure that their transplants have received the recommended copper spray program in the greenhouse.  In the field, start to apply the copper bacteriacide within 7 days after transplanting.  Apply at least 3 applications at 7-day intervals to keep bacterial disease from gaining a foothold in the crop.  This protects new growth and replenishes the greenhouse copper that is washed off by rain and dews.  If weather conditions are ideal for bacterial disease (ie. if there is a lot of wet weather), you may want to continue applications until early fruit set.

Growers’ decisions on extending the copper sprays may also depend on the end-use of the tomatoes (how many blemishes can be tolerated on the fruit) and if the plants are under other stresses that may weaken them and make them more susceptible to disease.

The key to this strategy is early prevention and control of bacterial diseases, before the population has a chance to build.  Since we cannot predict the onset or severity of bacterial disease, these early prevention and control strategies should be a part of your production system every year.  Once bacterial disease symptoms are present, it is too late to start to think about control.  Note, however, that most bacterial speck populations in Ontario are copper-resistant.

Many research trials across North America have shown that tank-mixing mancozeb with copper enhances bacterial disease control.   A new product for bacterial disease management, Actigard, has now been registered in Ontario, giving growers another tool to protect their crop.

How Does Actigard Work?  Actigard contains the active ingredient acibenzolar-S-methyl (ASM). ASM is a chemical that triggers the plant’s own defense system to produce proteins or structures that give the plant the ability to suppress pathogens that cause disease. Normally, these defense mechanisms are only triggered when the plant detects the presence of a pathogen. The reaction of the plant to applications of ASM is referred to as systemic acquired resistance, and is beneficial because the plant’s defense system is activated before an infection occurs.

Does Actigard Work?  Numerous trials have been conducted across North America and in most cases have demonstrated that Actigard is effective at reducing the severity of bacterial spot and bacterial speck on both processing and fresh market field tomatoes. For example, research completed in Ohio in 2001 demonstrated that spray programs including three applications of Actigard and five applications of Kocide + Dithane resulted in lower levels of bacterial spot symptoms on foliage than the nontreated control, whereas eight applications of Kocide + Dithane did not (Miller et al, 2002). In 2004 another trial completed in Ohio demonstrated that six applications of Actigard were more effective at reducing the percentage of diseased foliage than ten applications of Kocide + Manzate (Lewis Ivey et. al., 2005). Field evaluations of Actigard were also completed by Dr. Diane Cuppels at Agriculture and Agri-Food Canada in London, Ontario, in the late 1990s. The number of bacterial spot lesions on tomato leaves was 68 per cent lower than the nontreated control, and the number of bacterial speck lesions 29 per cent lower than the nontreated control (Louws et. al. 2001). Note that a number of these trials used different rates of Actigard than those on the approved Canadian label, and the spectrum of pathogens causing bacterial spot in Ontario is reported to have shifted to Xanthomonas gardneri (bacterial spot Group D) after this work was completed.

Will Actigard Cause Yield Losses?  Prior to using Actigard, it is important to thoroughly review the label. When you do this, you will notice that it contains a warning about potential yield losses. We reviewed several research reports summarizing yield data from over 45 field trials in North America, and a large majority of these trials reported no yield losses associated with applications of Actigard. In three cases, yield was lower than the standard treatments in the trial, and in one other case, yield was lower than the nontreated control.

Ideas for Incorporating Actigard into Your Bacterial Disease Management Program

  • Consider Actigard as a supplement to current practices, as opposed to a substitute. Bacterial disease control remains challenging, and Actigard is likely not a silver bullet solution. Maintain diligence with your current bacterial disease management program.
    • Tank-mixing Actigard with a fixed copper product registered for bacterial spot and speck control will mean you are targeting the problem from two angles. Fixed copper directly targets the pathogens that cause disease, while Actigard boosts the plant’s defense system.
    • Use common sense and avoid applications of Actigard if stressful conditions are anticipated, such as herbicide injury, cold weather, or hail. These stress factors in combination with an application of Actigard may have negative effects on plant growth. It is also a good idea to wait until 7 days after transplanting before making an application of Actigard in order to avoid plant stress.
    • Actigard is not registered for greenhouse use.

A more detailed review on Actigard was presented at Tomato Day in Leamington.  The presentation can be found at http://www.opvg.org/tomato-day.aspx. For more information on tomato bacterial diseases, see the OMAFRA factsheet “Bacterial Diseases of Tomato” at http://www.omafra.gov.on.ca/english/crops/facts/05-069.htm.

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