Keep bacterial diseases away from vegetable transplants

Know the diseases that infect your crops and how to manage them.

Bacterial diseases can infect tomatoes and peppers resulting in blighting. Not all spotting on the foliage is caused by fungi. It is important to distinguish between spots caused by fungi and bacteria because disease management differs.

On tomato transplants, three bacterial diseases can be problems and include bacterial canker, bacterial speck and bacterial spot. Peppers are affected by bacterial spot only. Of the bacterial diseases that cause problems on tomatoes, bacterial speck is probably the easiest to identify because of the small, dark-brown spots surrounded by a yellow “halo” that occur on the leaves. Bacterial spot that occurs on tomato and pepper is not as easy to identify as bacterial speck. Bacterial spot disease results in spots or blotches on the leaves and stems. These are larger than those caused by bacterial speck. Symptoms of bacterial canker on tomato transplants include small, tan “blister-like” lesions on the leaves and petioles and progress to form brown streaking and cankering. A diagnosis from an Extension educator or other knowledgeable professional is often warranted to separate symptoms of bacterial diseases from symptoms caused by fungi or other causes.

Close-up of bacterial canker on stem and leaves of a tomato seedling.
Photo A. Close-up of bacterial canker on stem and leaves of a tomato seedling.

Bacterial canker infection in a tomato flat.
Photo B. Bacterial canker infection in a tomato flat.

Bacterial speck on foliage of tomato seedlings.
Photo C. Bacterial speck on foliage of tomato seedlings.

Bacterial spot on foliage of tomato.
Photo D. Bacterial spot on foliage of tomato.

Bacterial spot on foliage of pepper.
Photo E. Bacterial spot on foliage of pepper.

Tomatoes with bacterial diseases should be immediately removed from the greenhouse and destroyed. In addition, tomato seedlings immediately adjacent to those showing symptoms should also be removed and destroyed. In some situations, all tomatoes within a block or greenhouse will have to be destroyed. Although epidemics may seem to appear overnight, chances are it began in just a few plants and progressed unnoticed for a couple of weeks. Plug sheets containing infected transplants should not be reused. Removing infected transplants from the greenhouse is the most critical component of managing bacterial diseases once they’ve been introduced.

Bacteria move readily in a film of water and can spread through splash droplets. It is important to water plants early enough in the day to ensure that the foliage dries completely by evening. Good ventilation, circulation and low relative humidity are also important in helping to maintain dry foliage. Clipping, pruning or any other type of injury provides a means for the bacteria to enter the plant and should be avoided.

Until recently, growers have had to manage bacterial canker as it occurred in the field. Our research team approached this problem by testing fungicide applications to transplants while in the greenhouse. The greenhouse was targeted because the spread and increase of bacteria is favored by the wet, humid conditions of the greenhouse and the close spacing of tomato transplants. Multiplication and spread of the bacterium is less likely in the field because of the lowered relative humidity and increased plant spacing. Also, it is more economical and efficient to spray transplants while in the greenhouse than to spray plants once placed in the field.

We focused on the health of tomato transplants because it has been our observation that establishing a field with transplants that are infected with the bacterium responsible for bacterial canker results in devastating yield losses. Transplants can be infected while in the greenhouse, yet appear healthy at the time of planting in the field.

Applying a copper hydroxide product alone or in combination with a mancozeb fungicide at five-day intervals to transplants in the greenhouse once true leaves had emerged, even when a bacterial canker epidemic occurred, resulted in transplants that produced yields comparable to that of healthy plants. In our studies, these copper applications were not continued once the transplants were planted in the field.

For tomatoes, the efficacy of the copper fungicides may be enhanced by mixing them with a mancozeb-based fungicide. Although mancozeb does not have any action against bacteria, the combination of mancozeb + copper is considered by some to provide a synergistic action against these bacteria. This combination would also provide some control of the foliar diseases caused by fungi (such as Botrytis and Alternaria). While Agri-mycin alone or in combination with copper hydroxide was also effective in our studies, this product does not list greenhouse on its label. However, current interpretations indicate that this product can be used on seedlings in the greenhouse since the label does not prohibit this use. We have not determined whether a seven-day interval of bactericides affords the same protection as the five-day application interval that we tested. Applications of copper hydroxide or Actigard to transplants once in the field may be helpful in reducing bacterial occurrence and fruit spotting.

General guidelines

  • Dedicate operations for seedling or transplant production. Greenhouses that grow both tomato transplants and mature plants for fruit production are especially at risk of keeping diseases active in the greenhouse and available to infect new tomato seedlings.
  • Keep the relative humidity as low as possible – less than 85 percent - through heating and venting as appropriate.
  • Space plants to prevent pockets of high humidity from forming.
  • Use fans to move air and vent to exhaust moisture-laden air out of the greenhouse.
  • Scout seedlings two times each week to ensure that problems are detected early when corrective measures can be taken.
  • If disease symptoms are detected, remove affected plants including adjacent, healthy-appearing plants.
  • Water at a time of day when plants can dry quickly.
  • Apply fungicide preventively when weather conditions are favorable for disease (i.e. wet, humid).

Please note. Disease prevention and prompt diagnosis are key components in vegetable seedling production because there are relatively few fungicides registered for controlling diseases on these crops. As long as the greenhouse use is not prohibited and the specific vegetable is listed on the label, the fungicide can be used in the greenhouse.

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