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Bacterial spot problem for vegetable growers

Bacterial spot, caused by Xanthomonas campestris pv. vesicatoria, is one of the most destructive diseases of tomatoes and bell peppers in the Southeast. The disease perennially causes problems in greenhouses and fields each year.

Control options for this disease include rotation, use of resistant varieties (pepper only), disease-free seed, and the extensive use of copper fungicides that are often tank-mixed with maneb.

However, new races of this pathogen continue to develop that are able to overcome available host resistance and commercial resistance has not been made available in tomato.

Bacterial spot also can become insensitive to copper, which renders remedial control ineffective. Whenever we test isolates of bacterial spot routinely in the Tifton Plant Disease Clinic, about 50 percent of the isolates are insensitive to copper alone.

Most isolates are sensitive to copper combined with maneb or mancozeb, but more and more we are seeing isolates that are resistant to that combination.

Actigard (acibenzolar-S-methyl) is a plant defense activator that has been successfully used to suppress losses to bacterial diseases of tomatoes. However, this product is not used on peppers because of phytotoxicity problems.

Recently, viruses that attack bacteria (bacteriophages) have been shown to suppress the spread and severity of bacterial spot outbreaks and are being tested for commercial use (commercial name Agriphage). However, bacteria soon become resistant to these phages and new phage strains have to be continually developed to deal with the resistance development.

Also, phage is readily inactivated by exposure to sunlight which reduces residual activity and presents spray timing issues.

Recently, the antibiotic kasugamycin has gained more interest since it was granted an import tolerance on food coming in to the United States from other countries where it is labeled.

This antibiotic may soon be made available to U.S. growers but should not be used too heavily since bacteria develop resistance to antibiotics quite readily.

Hopefully, growers will remember the past lessons learned with antibiotic resistance development to streptomycin and tetracycline antibiotics and rotate and/or tank mix kasugamycin should it be made available.

None of the above-mentioned materials work well alone, but combinations of them may hold the key to greater bacterial spot suppression and management of resistance to the control options is more likely to be successful if options are used in alternating patterns or in combination.

Even with more resistant varieties and combinations of bactericides, bacterial spot problems will continue to confound plant pathologists and growers as long as infested seed and transplants are being sold.

Using planting stock, whether seed or transplants, that are already infested with a pathogen is the single most important factor for inciting many of our bacterial outbreaks in vegetables.

Using infested planting stock bypasses almost every cultural and sanitary practice growers may employ, which puts more pressure on chemical sprays to provide suppression.

Growers may wish to use hot water treatments that work well to kill bacteria on seed, but these are difficult to conduct due to more seed being coated with commercial fungicide mixtures and hot water seed treatment may also affect the germination of some seeds.

It will take a combined effort between growers, the seed industry, and university researchers and Extension personnel to develop and maintain durable control measures for bacterial spot of peppers and tomatoes.

If we continue to rely on the “one newest tool” until it becomes ineffective, bacterial spot will stay at the top of the list for troublesome pathogens on vegetables.

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