Copper resistant bacterial spot strains found in Indiana

Published: March 1, 2011

Author: Dan Egel (Purdue University, IN)

In results confirmed in our laboratory late last year, several strains of the bacterium that cause bacterial spot of tomato and pepper were found to be resistant to copper. That is, the fixed copper products such as copper hydroxide and copper sulfate used for management of bacterial spot may not be effective management tools on these strains.

The strains of the bacterial spot bacterium resistant to copper are from both tomato and pepper. (Strains from pepper usually do not affect tomato and vice versa; the strains of the pathogen from bacterial spot of pumpkin do not affect peppers/tomatoes and vice versa.) One strain isolated in Indiana from tomato as far back as 1996 was resistant to copper. On the other hand, strains both resistant and sensitive to copper were isolated from tomato in 2010. Resistant strains of bacterial spot have been found on peppers, fresh market tomatoes and processing tomatoes. Bottom line— it is impossible to know whether the stains of bacterial spot in your tomatoes or peppers are resistant without testing them.

These results are not surprising. Strains of copper resistant bacterial spot pathogens have been known from Florida for decades. However, copper resistance has not been confirmed in Indiana until now.

Bacterial spot of tomato and pepper has been known in Indiana since 1921. Indiana was the 1st state in the U.S. to report this disease. Management of this disease has concentrated on applications of fixed copper applications. With the first report of copper resistant strains in Indiana, growers should review their control measures. Growers should assume the strains of bacterial spot in their fields are resistant to copper.

Management guidelines for bacterial spot:

• Fixed copper - growers with strains of the bacterial spot pathogen resistant to copper may have to increase the frequency of copper applications to overcome the level of resistance in bacteria. This is especially true during periods of hot, rainy weather.
• The use of mancozeb products (e.g., Dithane®, Manzate®, Penncozeb®) to accompany applications of fixed copper may allow more copper to become available on the leaf surface, thus overcoming the level of copper resistance that exists. Mancozeb products are not labeled on peppers.
• The product Actigard® may help to lessen the symptoms of bacterial spot by inducing the plant to increase its resistance to disease. This increase in resistance is not related to copper resistance. Actigard® is not labeled on bell peppers. Follow the Actigard® label closely to avoid yield loss due to the application of this product.
• Products with the active ingredient streptomycin (e.g., Agrimycin®, Firewall®) are not affected by copper resistance. Applications of this product in the greenhouse at least once will help to manage bacterial spot. Streptomycin products are not labeled for use in the field.
• Serenade® is a biological product labeled for use on tomatoes and peppers for bacterial spot. Some research suggests that this product, used in alternation with fixed copper products, will help to manage bacterial spot. Serenade® is unaffected by copper resistance.
• The use of a virus disease of the bacterial spot strains is another option. AgriPhage® is a product that uses a virus disease of bacteria to kill the strains that cause bacterial spot. The use of such a product requires one to send in plant samples to the manufacturer so that they can customize the product for your field. Contact me for more information about AgriPhage®.
• Some pepper cultivars have resistance to some combination of races 1 through 5 of the bacterial spot pathogen. The more races the cultivar is resistant to, the better the chance of beating bacterial spot. However, any resistance may be overcome. In 2010, a race of pepper, race 6, was found in central Indiana. This race would have overcome any known resistance in commercial cultivars.