Bacterial spot of tomato is one of the most serious diseases facing tomato growers in Indiana. As described at Vegetable Diseases in Greenhouses (PDF), bacterial spot is more of a problem for field tomatoes than for greenhouse tomatoes. Symptoms and management of bacterial spot are described briefly at Bacterial Spot of Tomato and Pepper (PDF). A more detailed version of this article is found at An Update on the Use of Copper Products for Managing Bacterial Spot of Tomato (Blog Post). This article will discuss why copper products have become less useful in the control of this important disease and options for managing bacterial spot of tomato.
Copper products have been used for many years to help control bacterial spot of tomato. However, some strains of the bacteria that cause this important disease are resistant to copper—that is, the bacteria have mutated to a form that is no longer sensitive to copper. Some of the techniques used to increase control with copper have included increasing the application frequency of copper products, increasing the amount of copper applied and mixing copper with the product mancozeb (e.g., Dithane®, Manzate®, Penncozeb®) to increase the amount of copper available on the leaf surface.
The fact that many strains of the bacterial spot pathogen had become resistant to copper products 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.
While copper may not be as effective against bacterial spot as it once was, the lack of alternatives to copper makes this extension worker hesitant to change recommendations to exclude copper. 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.
Since copper products alone will not control bacterial spot of tomato, what other options do we have? Chemical control. It is impossible to control bacterial spot of tomato without cultural controls such as crop rotation, sanitation etc. However, this article will concentrate on 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.
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. In general, I do not recommend the application of hydrogen dioxide products in the field for control of bacterial spot. Do not substitute hydrogen dioxide for copper or streptomycin or Actigard®. But do not mix copper and hydrogen dioxide.
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.
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.
The fungicide Tanos® (common name of active ingredients, famoxadone, 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.
I welcome any comments or questions about bacterial spot of tomato.