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Bacterial Canker of Tomato found in MN Gardens

M. Grabowski, UMN Extension

Bacterial Canker of Tomato - Heinz USA ,
Bacterial canker of tomato caused by the bacteria Clavibacter michiganensis sbsp. michiganensis (CMM), is an  emerging plant disease in Minnesota. Although this disease occurs regularly in other states, the first reported incidence of bacterial canker in Minnesota occurred just a few years ago. Bacterial canker can be a serious problem for vegetable farmers, especially tomato growers that use high tunnels or greenhouses to grow their crop. In addition, the bacteria can infect tomato seed under the seed coat and result in spread of the pathogen to new areas.

To determine just how wide spread this disease is in Minnesota, the Minnesota Department of Agriculture has been looking for CMM as part of their pathways survey for new and emerging invasive plant pests. In 2015, the MDA surveyed 90 community gardens, small vegetable farms and community supported agriculture farms. The pathogen that causes bacterial canker was found in Blue Earth, Carver, Dakota, Hennepin, Nicollet, Ramsey, Todd, Waseca and Washington counties.

What do plants infected with bacterial canker look like and how do I distinguish this disease from other common tomato disease problems?
The best way to determine if a plant is infected with bacterial canker is to send a sample to the UMN Plant Disease Clinic. The symptoms of the disease can vary greatly and it is easy to confuse symptoms of this disease with other problems.

M.A. Hansen, VA Polytech. Institute,
Infected seedlings may have few or no symptoms or may completely wilt and die. Older infected tomato plants have tan to yellow patches between veins on leaves and dark sunken streaks along leaf veins and petioles. If tomato leaves become infected from the edge of the leaf, the leaf edge turns dark brown and a yellow border often separates the killed leaf tissue from healthy green tissue.  Infected peppers have yellowing of the leaves that fades gradually into the green healthy tissue.

In severely infected tomatoes, the bacteria enter the main stem and a dark discoloration can be seen inside the stem if it is cut in half lengthwise. The stem often cracks and splits, creating a dark brown streak along the stem that may ooze sticky yellow liquid in wet weather. As the stem becomes more severely infected, leaves wilt, often first on one side, and eventually the entire plant collapses.

Fruit may develop a bird’s eye spot - a small raised spot with a white ring around it.

Where did bacterial canker come from and how does it spread?
The most common way that CMM enters a garden is through contaminated seed and transplants. It takes only one infected seed to bring in the pathogen. Once in the garden, CMM can be spread on hands and tools, and in splashing rain or irrigation water. The pathogen can survive in debris from infected plants and on weeds in the tomato family (solanaceae) for several years. It can also survive on contaminated tools and equipment for several months. Once established in a garden, CMM can be a difficult pathogen to eradicate and may be found in the garden again next year.

What should I do if bacterial canker is found in my garden?
CMM may be confirmed in your garden by the UMN Plant Disease Clinic. If the disease is identified early in the growing season, completely remove infected plants to prevent spread of the disease to nearby healthy plants. Infected plants should be buried or composted on site in a compost pile that heats up and results in complete breakdown of the plant material.

Bird's eye fruit spot of Bacterial Canker - Heinz USA ,
At this time, it is NOT recommended to bring infected plants to municipal compost sites. This is to prevent movement of the pathogen to areas of the state where it is not currently found. It should be noted that not all compost facilities achieve the heat necessary to completely kill the pathogen.

Tools should be soaked in a 1:9 solution of germicidal bleach in water for at least 5 minutes after working with infected plants. Gardeners should avoid working in plants when they are wet as the bacteria are easily spread at this time. Sprinkler irrigation should be avoided as splashing water will spread the pathogen. Use drip irrigation or soaker hose whenever possible.

At the end of the season, any infected plants should be buried at the site or composted on site. Cutting the plants into smaller pieces will speed up the breakdown process. Expect the bacteria to survive 2-3 years in this infected plant debris. Tillers and tractors can move infected plant debris through the garden or to new areas. To avoid spreading CMM, use equipment in known infected areas only after other areas have been tilled. Clean all soil and plant debris from the equipment and wash with a 1:9 solution of germicidal bleach in water before using the equipment in a new area.

G. Holmes, CA Polytech. State Univ.,
Avoid planting tomatoes or peppers in the area where CMM was found for 3 years. During this period, remove all weeds from the tomato family, like nightshade, as well as any pepper or tomato plants that grow from previous season’s fallen fruit.

Throw away wooden stakes, twine, bamboo or other structures made of natural material used in the garden. These are difficult to clean because they are extremely porous, making it difficult for disinfectants to reach the bacteria inside.

Tools, metal tomato cages or other equipment made out of metal or smooth material can be cleaned with a 1 to 9 solution of germicidal bleach and water.

  • Remove all soil and plant debris from tools and trellises. 
  • Wearing water proof gloves, mix 1 cup bleach with 9 cups water.
  • Soak all tools, tomato cages and other equipment in the bleach solution for at least 5 minutes.
  • Rinse all tools and equipment with clean water and dry before storing.

Do not save seed from infected plants or from tomatoes or peppers grown in a CMM-positive garden.

Are fruit from plants infected with bacterial canker safe to eat?
There is no record of CMM causing direct harm to a human. Gardeners should talk to their doctor if they have any concerns related to their health as many factors affect how individuals respond to microorganisms in the environment.

The major concern with plant pathogens infecting tomato fruit is that the infection can change the pH (acidity) of the fruit. This means that diseased fruit should never be used for canning because the pH plays an important role in keeping out other harmful microorganisms. The fruit can be cooked and eaten, cooked and stored, or frozen, cooked and eaten. The change in pH can let other harmful microorganisms enter the fruit so cooking is a best practice to kill microorganism that might be present prior to eating.

What can I do to prevent bacterial canker in my garden?
CMM is most commonly introduced on infected seed or transplants. Purchase seed from a reputable supplier. Do not save seed from infected plants. Seed from unknown sources should be avoided. If questionable seed is unavoidable, seed should be soaked in 1:4 solution of bleach in water for 1 minute and then rinse in water for 5 minutes prior to planting.

Purchase healthy transplants from a reputable grower. Avoid transplants that have been pruned or cut back as this procedure easily spreads CMM.   

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