Copper sprays have long been used to control the foliar bacterial diseases of tomatoes — too long, perhaps.

As with many other pesticides, overuse can lead to resistance development in the target organism, bacterial spot. 

The resistance to copper that causes Florida tomato growers control problems has been found in Tennessee — all over Tennessee.

A survey of tomato fields conducted by a University of Tennessee graduate student, Jonathan Mixon, found resistance to copper in 95 percent of the bacterial spot isolates collected.

The only isolates that were sensitive to copper came from some fields in western Tennessee in which copper use was not as intensive.

All the samples from the major tomato-growing regions in eastern Tennessee and the Walden’s Ridge area were resistant. In addition, two of three home garden isolates were resistant. 

Jonathan’s goal was to determine whether copper resistance occurred in any of the foliar bacterial pathogens of tomatoes — bacterial spot, bacterial speck or bacterial canker.

All the fields he visited had bacterial spot. However, the hot, dry weather of the two years in which the survey was conducted, 2010 and 2011, was not favorable to bacterial speck and bacterial canker, which prefer more moderate temps.

Bacterial speck was not found and bacterial canker was found in only one field and one home garden. The bacterial canker isolates were sensitive to copper.

Although resistance to copper has never been reported in bacterial canker, control of this disease by copper is usually somewhat weak.

What does this widespread resistance to copper in Tennessee mean?

First, it means this situation probably occurs in many other areas. Plants and plant products are actively involved in interstate commerce, and could carry resistant bacteria with them.

Even if it were not for this, Tennessee growers are not unique in their copper usage habits. Sensitive bacteria can convert to resistance wherever copper is used intensively.

These results also mean that much of the control growers thought they were getting from copper sprays was due to nothing more than dry weather. Those flare-ups you may have noticed after brief rainy periods are a harbinger of what may come after extended rainy periods. 

If you use copper products, tank-mix with mancozeb, as this improves the level of control by copper.

The plant activator, Actigard, operates against both copper-sensitive and resistant strains.

Actigard can cause slight yield suppression.

However, research in Florida indicates that by capping the rate at 0.50 ounce per acre, rather than the labeled 0.75 ounce, there is no yield suppression and disease control is still acceptable. 

Another alternative to copper is the bacteriophage product AgriPhage, for spot and speck control, and its counterpart AgriPhage CMM, for canker control.