Tomato disease management and spray adjuvants

From ONvegetables in The Grower, February 2013

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

Bacterial disease is an annual problem in field tomato production in the Great Lakes region.  The diseases are difficult to manage and weather conditions often favour the disease over the crop.

There has been some concern that spray adjuvants could be helping bacterial pathogens to penetrate the protective layers of the tomato leaf or fruit, thereby contributing to disease problems.  Spray adjuvants, such as spreader-stickers, are added to the spray tank to improve the performance of certain pesticides.

At the Tomato Disease Workshop, held in Ohio in October, Dr. Sally Miller of Ohio State University presented the results of a study on the effects of adjuvants on tomato bacterial spot.

In this study, several bacterial disease control products were applied with or without a non-ionic surfactant.

This surfactant, Activator 90, is in the nonylphenol family, similar to products like Agral 90 or Ag Surf that are used in Canada.

Treatments were:

• Kocide
• Kocide + surfactant
• Cuprofix
• Cuprofix + surfactant
• Actigard
• Actigard + surfactant
• Actigard + Kocide + Kasumin
• Actigard + Kocide + Kasumin + surfactant
• Surfactant
• Kocide + Manzate
• Non-treated control

The lowest level of foliar bacterial disease was in the plots treated with Kocide plus Manzate, a standard treatment used by Ontario growers.  Disease levels were also lower than the untreated control in the Cuprofix, Cuprofix + surfactant, Actigard + surfactant, and Actigard + Kocide + Kasumin + surfactant treatments.  In most cases, disease levels with the surfactant added were numerically lower than disease levels without the surfactant, although the differences were not statistically significant.  When the surfactant was used alone, with no bactericide or plant defense activator, disease levels were not significantly different than the untreated control.

One might argue that these surfactants were applied with bactericides, and thus did not increase disease.  In Ontario tomato production, adjuvants would most often be used with certain herbicide applications, rather than with bactericides.  However, in the treatment where the adjuvant was applied alone, there was no increase in the level of disease. If the adjuvant was helping the pathogen to invade the leaf, this is where the difference should have really showed up, as there was no freshly-applied bactericide protecting these plants.

This research should reassure tomato growers about the use of adjuvants.  It is good to keep in mind that when a pesticide label specifies the use of an adjuvant, efficacy of the product could be compromised if it is not used as directed.  In some cases, there is essentially no efficacy without the use of the recommended adjuvant.