This disease is becoming more prominent partly due to the increasing frequency of severe weather events that favor disease development as well as shorter rotations between tomatoes due to the economic value of the crop.
Foliar lesions are initially small, dark brown or black and circular and may be surrounded by a yellow halo. As the lesions expand, they coalesce and portions of the leaf or the entire leaf will turn yellow and die. Lesions on the pedicals can cause flower abortion. Lesions will develop on immature fruit as a result of infection of the fruit hairs. The fruit lesions are initially small, dark brown and raised. As the fruit ages, the lesions will increase in diameter and have a scabby or corky appearance.
Young bacterial spot foliar lesions can be difficult to distinguish from those of early blight or Septoria leaf spot however as the lesions expand, early blight lesions will develop concentric rings while lesions from Septoria will become tan in the center will small black dots called pycnidia. Early blight fruit lesions will develop similar concentric rings as on the leaves while Septoria will not cause fruit symptoms.
The primary sources of bacterial inoculum are infected seed, infected crop debris and contaminated equipment (tools, greenhouse structures, stakes, etc.). The bacteria are splash dispersed through overhead irrigation and during wind-driven rain events. They enter the plant through both natural openings and wounds caused by severe weather, insect or mechanical damage. The optimum temperature for infection is between 75 and 86°F while disease development is favored by temperatures that fluctuate between 68 and 95°F. Warmer night temperatures between 72 and 82°F will favor disease development while cooler night temperatures around 61°F will suppress disease progress.
The disease is caused by a group of xanthomonad bacteria once called Xanthomonas campestris pv. vesicatoria and divided into four groups (A, B, C, and D). Now, the groups have been taxonomically reclassified into four different species including X. euvesicatoria, X. vesicatoria, X. perforans and X. gardneri. X. euvesicatoria is the one that also affects peppers.
When managing bacterial spot as well as other bacterial diseases, it is important to keep in mind that bacteria readily multiply and spread under warm wet conditions; much faster than fungal pathogens. Once the bacteria land on a plant they then need a natural opening or wound to infect unlike a fungal pathogen that can directly penetrate the plant tissue. Therefore, in order for protectant bactericides to be effective, they must be applied and come in contact with the bacterial cells prior to them entering the plant. This is in contrast to some fungicides that can stop the fungal infection process even after the pathogen has initially entered the plant. Unfortunately, our ability to detect bacterial pathogens associated with the seed and/or transplants is not below the threshold to cause disease so an integrated approach that focuses on excluding bacteria from production system is critical for successful bacterial disease management. Since it is primarily the immature fruit that are more susceptible, minimizing potential spread from the seed through the main fruit set will minimize potential harvest losses.
Also plant stress whether due to environment or resulting from soil compaction, weed and pest pressure and field operations can also play a large role in plant susceptibility to bacterial pathogens. Implementing strategies to optimize planting and growing conditions can help also help reduce losses.
Select pathogen-free seed from a reputable source
Although often easier said than done, it is important to purchase the highest quality seed possible. The seed extraction process will not reliably eliminate bacteria from the seed so treating the seed either with a chlorine bleach to disinfest the seed surface or a hot-water seed treatment to disinfest the seed surface as well as eliminate bacteria that may be under the seed coat may be necessary. In order to not reduce seed germination and viability it is important to follow established seed treatment protocols. With funding from the Pennsylvania Vegetable Marketing and Research Program/Pennsylvania Vegetable Growers Association and the Northeast IPM Program, four sets of hot-water seed treatment equipment have been purchased and are available for use by growers in Pennsylvania (check with your local extension educator about availability).
Sanitation during transplant production
Sanitation is an essential component to transplant production. Whether you are growing your own transplants or purchasing from a supplier, sanitation practices need to be in place to reduce potential losses to bacterial diseases. Some of these practices include:
- Removing all plant material including weeds from the greenhouse between crops.
- Using sterile potting mix and new trays, if possible. If re-using trays, sanitizing them with a disinfectant once all the organic matter has been removed. Keep in mind that disinfectant products will be neutralized and become ineffective in the presence of organic matter.
- Disinfect tools and equipment and greenhouse surfaces before the growing season. Wood can harbor bacterial pathogens between seasons so minimizing the use of wood in the greenhouse is important.
- Avoid co-mingling seed lots (as well as tomatoes and peppers) both during seeding and within the greenhouse to minimize potential cross-contamination.
- Minimize handling of the plants and movement within the greenhouse as much as possible.
- If symptoms are observed, remove the entire flat and adjacent flats to prevent further spread. Many of the seedlings may be infected but are not yet showing symptoms and may not develop symptoms until planted in the field.
- During transplant production minimize leaf wetness through timing of watering, managing relative humidity and ventilation. Either bottom watering or using low pressure nozzles for watering will minimize potential damage to the plant and ports of entry for the bacterial. Also do not handle plants when they are wet or allow them to drop on each other when preparing for shipping.
- In the field, follow a minimum three year crop rotation out of solanaceous crops including pepper. The bacterial spot pathogen cannot survive in the soil in the absence of crop residue. Also manage any solanaceous weeds or volunteer that can harbor bacterial spot between seasons.
- Minimize crop stress by promoting good soil drainage, adequate crop fertility and maximizing air circulation. Utilize drip irrigation whenever possible.
- Separate seed lots when possible to minimize potential spread if one seed lot is infested. Separate sequential plantings and work in the youngest planting first. Once the main fruit crop is set, late-season foliar symptoms and potential spread to the fruit are less problematic. Plowing down crop residue soon after harvest is complete will facilitate the decomposition of crop residue and reduce spread to younger successive plantings.
- In the field, fixed-copper based products are still the primary tool for managing bacterial spot in tomato. There is increasing concern about the potential development of copper resistance within bacterial populations in Pennsylvania as has developed in other production regions in the U.S. The current status of copper resistance in PA is currently not known but the collection and screening of select isolates is being planned for the 2015 growing season. In non-certified organic systems, copper can be tank mixed with mancozeb to enhance the bactericidial effect of fixed copper and this will also help manage any copper resistant strains, if present. An increasing number of research-based trials have demonstrated that when applied starting early in the season, Actigard 50WG (a.i. acibenzolar-S-methyl) can help reduce bacterial spot incidence and severity. This product works to triggers the plant's own defense system to produce proteins and other products that enable the plant to suppress pathogens. Typically, these defense mechanisms are only triggered when a plant detects a pathogen. Priming or the pre-activation of the plants defenses enhances the plants defense response. This is not a silver bullet and should be used in conjunction with an IPM program.
- In the greenhouse, bacteriophage (viruses that infect bacteria) can be used to reduce bacterial populations on the plant surface. These viruses are very specific to the bacterial species and when they come in contact, the virus injects its RNA into the bacterial cell where it replicates and eventually causes the bacterial cell to lysis or break open. Streptomycin-based products can also be used during transplant production to manage bacterial populations on the surface of the transplants.
In order to be effective, an integrated approach to the management of bacterial spot on tomato needs to be employed from seed selection through fruit production. Although a challenge to manage, understanding the biology and epidemiology of bacterial spot can help identify strategies to reduce losses in the field and can help explain why management efforts may have failed.