An Update on the Use of Copper Products for Managing Bacterial Spot of Tomato

For 100 years bacterial spot has been causing huge losses for tomato growers worldwide. For 100 years products containing copper have held the key to controlling this devastating tomato disease. As tomato growers enter their second century of dealing with bacterial spot, the question has become whether copper applications lessen the severity of bacterial spot-or perhaps even make the disease worse. This article will discuss bacterial spot of tomato, why copper products have become less useful in the control of this important disease and finish with options for managing bacterial spot of tomato with and without copper.

The first symptoms of bacterial spot one is likely to observe are small, less than 1/8 inch dark lesions on tomato leaves. The lesions may appear watersoaked, especially in the morning and are often surrounded by yellow (chlorotic) tissue. These lesions, whether found on leaves or stems, may coalesce to cause the loss of large areas of plant tissue. Loss of yield or fruit quality may result from foliage lesions or lesions that occur on fruit. For a link to the Purdue Tomato Doctor app, click here.

Today, copper products are used primarily to manage bacterial diseases. At one time, however, copper was perhaps the most important fungicide as well. The use of copper to control diseases of plants goes back to 1882 when Professor Millardet noticed that the use of a copper substance on grape leaves to deter thieves also protected the vines from downy mildew. While initially crude copper substances were applied to leaves, copper products were refined so that small particles of insoluble copper compounds could be applied, remaining on the leaves for a significant period of time. Such products became known as ‘fixed copper’ products.

Heavy metal elements, such as copper, were often the products of choice to manage plant diseases until the 1930’s and 1940’s when synthetic fungicides began to be produced. Since that time, it has been recognized that, in general, synthetic compounds are more effective against fungal diseases than elemental based products such as copper hydroxide. Bacterial spot of tomato is one of the diseases for which copper products has remained an important management tool.

The repeated application of copper products, however, did not always control bacterial spot of tomato. Scientists began to turn up evidence that the bacteria that cause diseases such as bacterial spot of tomato became resistant to copper over time. It seemed that the more copper applications that were made, the faster the bacteria became resistant to copper.

Since bacteria first evolved on earth billions of years ago, they have had to find ways to get rid of heavy metals like copper. When humans began to use copper to try to control bacterial diseases, genetic traits that allowed bacteria to overcome copper toxicity have become more frequent in the bacterial population. In 1986, Robert Stall of the University of Florida found that the trait that allows strains of bacterial spot pathogen to resist copper could be found on a small piece of DNA (known as a plasmid) that is easily transferred to other bacteria. Therefore, this important piece of DNA that codes for copper resistance could be easily spread within bacterial populations.

Although there was increasing evidence of copper resistance in strains of bacterial spot of tomato, copper products continued to be used as an important tool. Some of the techniques used to increase control with copper included increasing the application frequency of copper products, increasing the amount of copper applied and mixing copper with the product mancozeb to increase the amount of copper available on the leaf surface.

The fact that many strains of the bacterial spot pathogen were resistant to copper was bad news. Recently, however, even worse news was reported. The new evidence shows that, in at least some cases, the use of copper actually makes bacterial spot worse than not using any copper at all.

Why would the use of copper result in more bacterial spot of tomato? No one knows for sure. It is probably safe to assume that in such circumstances, almost all the bacteria present are resistant to copper. The use of copper therefore did not lessen the disease. Plus, it may be that the copper product eliminated all the beneficial micro flora (bacteria and fungi) on the leaf, making the leaf an excellent environment for bacterial spot.

At a recent Tomato Disease Workshop in Windsor Canada, whether or not to recommend copper products for bacterial spot was one of the topics discussed. Most extension workers will leave copper products in the list of recommendations for bacterial spot of tomato. The reasons are:

1. 1. One of the most prominent studies that show copper products may make bacterial spot of tomato more severe were conducted at the University of Florida (by Professor Gary Vallad). Weather conditions are near perfect for bacterial spot in Florida, perhaps better than anywhere else in the U.S. It is possible that with less conducive weather conditions (like in Indiana), copper products may be able to perform better.

2. 2. Because bacterial spot is so much more severe in Florida than anywhere else, more copper applications are used in Florida, leading to higher proportions of copper resistance in bacterial populations. In Indiana, for example, it is probable that the percentage of copper resistant bacteria is lower than in Florida. Observations suggest that copper products are still useful in Indiana.

3. 3. There are very few good answers to managing bacterial spot of tomato. While copper may not be the answer it once was, the lack of good answers makes this extension worker hesitate to cross copper products off the list. Indiana tomato growers will find that copper products are still one of the options listed in the Midwest Vegetable Production Guide for Commercial Growers, 2015 (expected in January).

Since copper products alone will not control bacterial spot of tomato, what other options do we have?

Cultural conditions

Although it has been said many times, many ways, crop rotation is an excellent method to lessen the severity of bacterial spot. Or, to put it another way, if you don’t rotate, bacterial spot will almost surely become a big problem.

Although there are no tomato varieties that are resistant to bacterial spot, varieties vary in susceptibility. Ask your seed company representative about bacterial spot and host susceptibility. Make your own observations about bacterial spot susceptibility in your field on your varieties.

Always choose tomato seed that have been tested for the bacterial spot pathogen. Consider treating seed for bacterial spot (see the MidwestVegetable Production Guide for Commercial Growers, 2015). Avoid, if at all possible, growing transplants of different varieties together in the same greenhouse. Scout transplant greenhouses carefully for symptoms of bacterial spot. Do not plant transplants from a greenhouse where bacterial spot has been found. Carefully clean and sanitize transplant greenhouses in between generations of transplants.

Water is necessary for bacterial spot of tomato to initiate disease and for disease spread. Any practice that lowers the amount of leaf moisture will lower the amount of disease. For example, overhead irrigation should be avoided in favor of drip tape. If overhead irrigation is unavoidable, do not irrigate in the evening when the tomato leaves will likely remain wet through the morning dew period.

Tomatoes grown to maturity in greenhouses or high tunnels often do not have severe symptoms of bacterial spot of tomato. The greenhouse covering will, for the most part, keep moisture from forming on leaves and prevent rains from splashing bacteria from leaf to leaf. Thus, growing tomatoes in greenhouses or high tunnels is one method to lessen the severity of bacterial spot. (However, diseases such as leaf mold, gray mold and white mold are often more prominent in a greenhouse than in a field.)

Chemical control

Products with streptomycin (e.g., Agri-mycin 17®, Firewall®, Harbour®) can be used in the transplant greenhouse (Streptomycin products cannot be used on field tomatoes). The use of streptomycin products will help to lower the populations of strains that cause bacterial spot, including those strains that are resistant to copper. It makes sense to use streptomycin products at least once and a copper product at least once in a tomato transplant house. If both applications are effective, the only strains remaining will be strains that are resistant to both copper and streptomycin-a small sub population hopefully.

Products with the active ingredient hydrogen dioxide (e.g., Oxidate®) are also labeled for bacterial spot in the greenhouse. Hydrogen dioxide can kill bacteria on contact, however, it has very little to no residual. That is, an hour or so after application, there will be no activity from these products. This is in contrast to copper or streptomycin products. However, there is no resistance to hydrogen dioxide. So, if one were to apply hydrogen oxide to a greenhouse full of tomato transplants, it might be possible to lower the populations of strains of the bacterial spot pathogen regardless of copper resistance. Due to the lack of residual of hydrogen dioxide products, if the decision to use this product is made, do not leave out a streptomycin or copper application. Rather, use a separate application of hydrogen dioxide. In general, I do not recommend the application of hydrogen dioxide products in the field for control of bacterial spot. (Some compounds of copper, streptomycin and hydrogen dioxide may be organically approved, that is, approved by the Organic Materials Review Institute-OMRI​).

Another product that has been used for management of bacterial spot of tomato is acibenzolar-S-methyl (trade name Actigard®). Acibenzolar (ASM) does not have any activity against bacteria or fungi. ASM is known as a systemic acquired resistance product. That is, it ‘tells’ the plant to turn on biochemical pathways that defend the plant from infection. ASM has been used with copper products to lessen the severity of bacterial spot of tomato. However, ASM can cause yield loss if used on tomatoes that are stressed due to drought or other environmental factors. The effectiveness of ASM will not be influenced by whether the bacterial strains are resistant to copper or not.

Serenade Max® has shown activity against bacterial spot of tomato. The action of Serenade Max is reported to be due to a protein component of the bacterial ingredient and to a systemic acquired resistance activity similar to that described for ASM. As with ASM, the activity of Serenade Max® is not related to whether the bacterial spot strains are copper resistant or not. (Serenade Max® is OMRI approved).

The fungicide Tanos® (common name of active ingredients, famoxadone plus cymoxanil) has been trialed for activity against bacterial spot of tomato. While the results have not always been positive, it might make sense to use Tanos® when one is trying to manage one of the fungal diseases on the Tanos label (for example anthracnose, early blight, late blight, Septoria leaf blight) and hope for some activity against bacterial spot as well. The effectiveness of Tanos®against bacterial spot will have nothing to do with possible copper resistance of the pathogen strains.

Conclusions

None of these management techniques, whether cultural or chemical, will be effective as a stand-alone technique for bacterial spot of tomato. Each tomato grower will have to work the management tools mentioned into an overall program. Such a program of bacterial spot management of tomato will include many of the cultural techniques mentioned. Most programs will use copper, Actigard® and work in some of the newer products such as Serenade Max® or Tanos®. The use of some type of streptomycin product as well as copper in the transplant greenhouse is a good idea.

Plant pathologists in industry, academia and government are working on alternative products that may be released someday. It is likely, however, that even these newer tools will only represent a portion of the battle that growers will continue to fight against bacterial spot of tomato.

Selected References

E.C. Large. 2003. The Advance of the Fungi. The American Phytopathology Society. An account of Professor Millardet and the discovery of copper as a fungicide is just one of the great stories in this book.

Boyd, V. 2014. Rethinking Copper. Citrus and Vegetable Magazine, pages 10-11. This is an account of some of Professor Gary Vallad’s excellent work at the University of Florida about copper making bacterial spot more severe.

Stall, R.E., D.C. Loschke and J.B. Jones. 1986. Linkage of Copper Resistance and Avirulent Loci on a Self-Transmissible Plasmid in Xanthomonas campestris pv. vesciatoria. Phytopathology 76:240-243. This manuscript demonstrates that the trait that controls whether a bacterium is resistant to copper may be located on an easily transmissible piece of DNA.

Marco, G.M., R.E. Stall. 1983. Control of Bacterial Spot of Pepper Initiated by Strains of Xanthomonas campestris pv. vesciatoria That Differ in Sensitivity to Copper. Plant Disease 67:779-781. One of the finding of this manuscript is that the use of the fungicide mancozeb increases the amount of copper available.

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