Managing Bacterial Diseases of Tomato in the Field
Margaret Tuttle McGrath
Bacterial diseases in tomato are most effectively managed with a preventive program targeting the various potential sources of these pathogens. Implementing control measures before symptoms are seen is critical because time between infection and appearance of symptoms can be long. And, once established, bacterial diseases are very difficult to suppress. Infested seed, transplants, weeds, and crop debris as well as contaminated planting materials and greenhouse surfaces are all possible sources of the bacteria causing three important diseases affecting tomato in the northeastern US: speck, spot and canker. After implementing the practices described in ‘Controlling Bacterial Diseases in Greenhouse Transplant Production’, continue the management program in the field with rotation, sanitation, drip irrigation, and pesticide applications described in this article. There are a few roma-type tomatoes with resistance to speck.
Select a field that has not been used for producing tomatoes or peppers for at least two years and where solanaceous weeds and volunteer tomatoes have been controlled. Pepper is susceptible to bacterial spot, and to a lesser extent bacterial canker. A minimum rotation of three years is recommended for canker because bacteria causing this disease can survive in tomato stems, which decompose slower than leaves and fruit. Solanaceous weeds and volunteer crop plants can enable bacterial pathogens to survive during the rotation period, therefore their control is important.
Sanitation in the field includes cleaning transplanting equipment, using clean stakes for trellising tomatoes and avoiding moving bacteria during field operations. Clean transplanting equipment between fields. Used stakes should be disinfected even if the previous crop did not appear to have any bacterial diseases. Wash off dirt, then disinfectant with a hydrogen dioxide product such as ZeroTol or a quaternary ammonium product such as Green-Shield. It is recommended that new stakes be used rather than re-using stakes if bacterial diseases, especially canker, affected tomatoes the previous year. Plants should not be handled when wet. Sanitation is especially important during pruning and trellising as bacteria can easily be moved on hands and tools, plus resulting wounds provide sites for infection. Gloves that can be disinfected routinely while working are recommended. Tractors and other equipment that brush against plants can also move bacteria.
Drip irrigation is recommended because splashing water from overhead irrigation can move bacteria and the resulting wet leaf surfaces provide these pathogens conditions needed to infect.
Pesticides registered for managing bacterial diseases in tomato include several formulations of EBDC fungicides (mancozeb, maneb, and ziram), coppers (eg Kocide, Champ, Cuprofix), Actigard and Tanos. For a complete list of EBDC and copper products see http://vegetablemdonline.ppath.cornell.edu/NewsArticles/Tom_LabeledRts.html. The new products, Actigard and Tanos, are suitable for use with other products, rather than alone, thus they are recommended with the standard program of a copper tank-mixed with an EBDC fungicide that has been in use for years. Fortunately the standard program is relatively inexpensive, costing only about $8.39/A/week based on $3.71 for Manzate 75DF at 1.5 lb/A and $4.67 for Champ at 1.3 pt/A. Cost increases to $12.10 when the full label rate of 3 lb/A Manzate is used because fungal diseases are also a concern.
Resistance to copper has challenged control of bacterial diseases. Control of copper resistant strains can be improved by increasing the quantity of available copper in spray solutions, which can be accomplished by using newer formulations of copper fungicides, using nonacidic water, and by tank-mixing copper with an EBDC and agitating the spray solution in the tank for 90 minutes before spraying. Frequent applications are also recommended. Copper resistance has not been examined in NY, but there have not been complaints of reduced control with copper suggesting that if resistant strains are present, control is not being affected substantially. Suspected cases of copper resistance should be promptly reported to extension specialists while plants are in production to enable testing.
Treatment should begin shortly after transplanting because bacteria can be present, multiplying on leaves, spreading through the crop, and even infecting crop plants for many days before symptoms develop. Symptoms may not be seen until plants begin to blossom, which could be as long as 84 days after infection. High pressure air blast sprayers should not be used because the force of the air used to move pesticide into the crop can move bacteria among plants.
Tanos is labeled for suppressing bacterial speck, spot, and canker as well as controlling several foliar fungal diseases in tomato. In university fungicide efficacy experiments, bacterial diseases were numerically less severe when Tanos was added on a 14-day interval to a weekly spray program of copper plus an EBDC fungicide. This trend has been consistent; however, treatments with Tanos have not been statistically significantly better than copper plus an EBDC alone. Tanos is labeled for use tank-mixed and alternated with other fungicides. It has a 3-day preharvest interval and can be applied up to 9 times at 8 oz/A, which costs about $10.30/A.
Actigard has a unique mode of action: it induces host plant resistance to speck and spot, but not to canker. Since Actigard does not have bactericidal activity, disease-causing bacteria can continue to survive and multiply on tomato plants sprayed with Actigard. Therefore, Actigard is recommended with copper plus EBDC fungicides. The label specifies a maximum of six applications and a 14-day preharvest interval. According to the directions on the label, applications should be made weekly beginning within one week of transplanting. There is potential for adverse effect on yield, therefore start with a rate of 0.33 oz/A applied at 30 to 50 gpa and increase the rate with subsequent applications to 0.75 oz/A applied at 70 to 100 gpa as described on the label. Actigard applied on a 14-day schedule in alternation with copper plus mancozeb has provided good control of bacterial speck without reducing yield in North Carolina, both in research and commercial fields. Although a weekly interval is specified on the label, Syngenta does support a 14-day interval. Our trials in Geneva, NY have also found Actigard to provide excellent control of bacterial speck without a reduction in yield. We used 0.75 oz/A applied at 100 gpa on a 7-day schedule. It takes at least three days for Actigard to induce plant defenses, so it is necessary to begin applications before symptoms appear on the plant. Actigard is not labeled for fungal diseases. The cost is about $22.32/A at 0.33 oz/A and $50.21 at 0.75 oz/A. If 8 weekly fungicide applications are made between transplanting and start of harvest in fresh-market tomatoes, applying Actigard on odd weeks at 0.33, 0.5, 0.75, and 0.75 oz/A will cost $156.22/A. Cost of the full 8 week pre-harvest spray program with Actigard and Tanos applied on alternate weeks along with copper plus an EBDC fungicide weekly is about $264.55/A.
Plants should be inspected routinely for symptoms to determine what bacterial diseases, if any, develop and to assess degree of control obtained. Symptoms of bacterial speck and spot on leaves are dark brown to black, irregularly-shaped spots typically with a yellow border. These spots coalesce and cause distortion of young leaves. Leaf spots also occur with bacterial canker, but the more typical symptom of this disease is brown leaf edges with a yellow inner border, and plants may wilt as a result of systemic invasion. Dark spots also develop on petioles, stems and fruit with these three bacterial diseases. Yield is reduced directly due to symptoms on fruit as well as indirectly due to loss of leaf tissue for photosynthesis. Spots on fruit often become raised, especially with canker. A white border typically occurs around fruit spots caused by canker. Click here for pictures: http://vegetablemdonline.ppath.cornell.edu/PhotoPages/Tomatoes/Tom_BactDiseases/Tom_BactPhotoList.htm.
Destroying a planting infected by bacterial canker is warranted when this disease becomes severe at an early stage in crop development and other plantings are symptom-free.
Crop debris should be incorporated into the soil immediately after harvest, then the equipment should be cleaned.
Diseases caused by fungal pathogens are also a regular occurrence in tomatoes. Therefore when inspecting a crop for bacterial diseases, look for symptoms of other diseases. While most products for bacterial diseases are also labeled for several fungal diseases, there are other products worth considering for an overall disease management program in tomatoes.
Please Note: The specific directions on fungicide labels must be adhered to -- they supersede these recommendations if there is a conflict. Any reference to commercial products, trade or brand names is for information only; no endorsement is intended.