by Matt Gibson
About Bacterial Canker
Bacterial canker is a bacterial disease that can affect tomato plants in both the home garden and in greenhouse environments. Once bacterial canker has infected a tomato plant, it can spread throughout the garden or greenhouse, resulting in a full-on epidemic that can drastically affect young tomatoes in the garden or every plant within a contained environment, such as a greenhouse.
Bacterial canker disease is caused by the bacterium Clavibacter michiganensis. The bacteria is erratic and incredibly destructive, attacking the foliage, fruit, and stems of tomatoes, peppers, and any plants from the nightshade family. First discovered in Grand Rapids, Michigan in 1909, the disease is currently active in tomato production areas all around the world, with annual outbreaks that are known to spread quickly, destroying entire tomato crops if left untreated. Fortunately, there are preventative measures that can be taken during every stage of production that can help gardeners avoid major losses from the bacterial disease.
Causes And Symptoms of Bacterial Canker
The organism Clavibacter michiganensis, which causes bacterial canker disease, can be found within infected seeds. The bacterium can survive for brief periods in the soil and in a greenhouse environment, but can live much longer in plant debris accidentally left behind after harvesting to affect future plantings.
The bacterial canker pathogen can travel long distances and can be introduced into new areas from infected seeds or transplants, and is easily spread between seedlings from unsanitized equipment, from worker’s hands and/or gloves, as well as from pruning and clipping infected transplants.
Just one infected seed can lead to a large
number of infected transplants, which can establish the presence of the disease
in fields and high tunnels. Symptoms may not be visibly present in transplants
at first, making it hard to catch before secondary spreading begins. Luckily
for commercial growers, infected transplants often die in the field and bring
very little crop destruction or secondary spread, limiting economic impact.
In tunnels and greenhouses, however, the disease spreads quickly and easily between transplants and older plants through staking and pruning practices, resulting in yield loss and severe symptoms. Surviving up to three years on plant debris and for months on equipment and garden stakes, the pathogen lingers and infects tomatoes planted in the same locations in the following seasons if crop rotation isn’t practiced. Bacterial canker also travels in water, such as rainfall, irrigation (especially overhead irrigation), or runoff, as well as on contaminated materials like garden supplies, gloves, clothing, and gardeners’ skin.
All tomato plants are susceptible to bacterial canker infection at every growth stage, from seedlings to mature fruit-bearing plants. Symptoms range from systemic to superficial, appearing first on young green fruit as small white spots with dark brown centers. Systemic symptoms tend to show up early in seeds or very young plants. Systemic symptoms include discoloration of leaf veins, wilting, and lesions on the stems and fruit of the plant.
Treatments and Control of Bacterial Canker
Unfortunately, bacterial canker is one of the toughest tomato diseases to manage and control. This is because of the difficulty in spotting the disease due to its wide range of symptoms. Also, the disease is incredibly infectious and no known chemical treatments can manage the symptoms effectively. Therefore, control methods and preventative measures are the only effective treatments. The following control methods are the best defense against the spread of bacterial canker disease:
- Planting disease-free or disease-resistant varieties. When growing from seed, be sure to purchase seeds that are certified disease-free. Never use any seed sources that may be known to have bacterial canker. Gardeners saving and using their own seeds is highly discouraged.
- If disease-free seeds are not available, treat seeds in order to kill off any pathogens or bacteria that may exist on the surface or interior of seeds. Soak seeds in an eight percent acetic acid solution, or a five percent hydrochloric acid solution. You can also use a hot water treatment, soaking seeds in hot water for 30-45 minutes.
- Practice regular crop rotation, waiting for a minimum of three years between planting tomatoes or any other nightshade plant in the same location. Since the bacteria is known to survive in plant debris, it is important to rotate your tomatoes with crops that cannot be infected with the canker bacteria. After three years of rotation, the disease should no longer be present in the location in question.
- Soil sterilization can be an effective control method to disinfect the soil in which infected plants were cultivated. When reusing greenhouses and garden beds, the soil must be sterilized in order to kill the bacteria living in the soil. Steam sterilization is the best method [https://homeguides.sfgate.com/sterilize-soil-steam-79037.html].
Common Questions and Answers About Bacterial Canker
How do you get rid of bacterial canker?
When you see the first signs of bacterial canker in your garden, remove affected plants as soon as you can, and dispose of contaminated plant debris either by burying them or adding them to hot compost. It’s vital to use one of these sterile methods of disposal to keep the disease from sticking around to spread throughout the garden or infect plants again next season. Not only plants showing signs of canker must be removed, but neighboring plants should be thrown out as well, as they can be infected but not yet showing signs of the disease.
The bacteria that cause bacterial canker can survive for years on infected plant debris. For this reason, it’s recommended that you not plant susceptible crops (tomatoes, peppers, or eggplants) in areas where bacterial canker has struck until three years have passed. Instead, use crop rotation and plant a type of plant that won’t catch bacterial canker in that spot. Nearby weeds can also provide bacterial canker with a hiding spot, so pull weeds carefully around zones of the garden that have struggled with bacterial canker.
Whenever you use gardening tools or supplies to work with infected plants, you must sterilize them in bleach or rubbing alcohol after use or before moving to healthy plants. This goes for large equipment like tractors or tillers as well. If you don’t sterilize contaminated tools, you risk spreading bacterial canker to healthy plants. Any wooden supplies (such as stakes and trellis) or equipment made of other porous materials (twine, string) must be thrown out once it’s exposed to bacterial canker. It’s impossible to eliminate the bacteria from porous materials once they’ve come into contact with the disease.
How do you prevent bacterial canker of tomatoes?
Because bacterial canker most often comes into a garden via contaminated seeds or transplants, the first step in preventing bacterial canker is to purchase your seeds or young plants from a trustworthy, high-quality supplier. Seeds should have undergone testing or treatment, and packaging should provide information about prevention measures. Look for certified pathogen-free seeds.
You can treat seeds yourself with hot water or bleach to eliminate the bacteria that cause canker if seeds are infected—but this process has undesirable consequences. Heat treatments can reduce the shelf life of seeds, slow down germination, and make warranties or guarantees from seed companies null and void. That’s why the best approach is to work with seeds or plants you know are disease-free and don’t have to treat. But if you need to treat seeds, you can either soak for one minute in a mixture of one part germicidal bleach to four parts water or soak for 25 minutes in water that’s 122 degrees Fahrenheit.
Make sure that the supplies you use in the garden are sterilized with bleach or rubbing alcohol before and after use and between plants or rows to prevent spreading disease. Tools and equipment of every size and material, from your hands or gloves to tractors and tillers, should be clean and sterile. Any porous material (like wood, twine, or string) that comes into contact with bacterial canker should be thrown out, as it’s impossible to get the bacteria out of porous materials.
Prevention also means minimizing the conditions that are best for bacterial canker. Water your plants as early in the morning as you can. Instead of watering from above, water from the base or use drip irrigation or a soaker hose. Don’t work in the garden when your plants are wet from rainfall, morning dew, or because you’ve recently watered the plants.
Another important way to prevent bacterial canker in the garden is to keep a watchful eye on your plants. That way, if an infection starts, you can take measures to stop the disease in its track to minimize damage and keep the bacteria from spreading. Check your plants for signs of bacterial canker and other illnesses regularly and meticulously.
How does bacterial canker spread?
Bacterial canker is spread by the bacterium Clavibacter michiganensis sbsp. michiganensis, which is sometimes referred to as CMM or Cmm. The disease usually finds its way to gardens through infected seeds or transplants. Bacterial canker also travels in water, such as rainfall, irrigation (especially overhead irrigation), or runoff, as well as on contaminated materials like garden supplies, gloves, clothing, and gardeners’ skin. CMM can also hide out in infected soil, on nearby weeds, or in plant debris from the previous season that’s left in the garden. Infected soil or plant debris can be blown into the garden by wind.
Is it safe to eat tomatoes with bacterial canker?
There are no reported cases of the bacteria behind bacterial canker (Clavibacter michiganensis sbsp. michiganensis, also called CMM or Cmm) making humans ill, according to the University of Minnesota Extension’s Michelle Grabowski. However, affected tomatoes should not be canned or otherwise preserved due to bacterial canker’s impact on the fruit’s acidity—and therefore its pH level. The pH level of preserved fruit is responsible for keeping harmful pathogens at bay. The UMN Extension recommends to cook or freeze and then cook tomatoes with bacterial canker if you wish to eat them.
However, the recommended course of action is to remove plants that show signs of bacterial canker as soon as the disease is apparent. Leaving infected plants in the garden increases the chance of bacterial canker spreading through your garden. The best approach is to immediately remove plants contaminated with bacterial canker, not allow fruit to ripen and harvest it for eating.
What causes bacterial canker in tomatoes?
Bacterial canker is caused by the bacterium Clavibacter michiganensis sbsp. michiganensis, sometimes abbreviated as CMM or Cmm. The disease is normally introduced to gardens via contaminated seeds or transplants. It is also spread through the movement of water (splashing rainfall, runoff, or irrigation) or by hanging a ride on garden supplies, tools, equipment, gloves, clothing, or gardeners’ hands. Bacterial canker thrives between 75 and 90 degrees Fahrenheit when the weather is moist and humidity is high.
What does bacterial canker look like?
Bacterial canker is most easily recognized by its hallmark “bird’s eye spots,” which are slightly raised spots on fruit ranging in color from yellow to brown that are surrounded by a white halo. The spots are very small—about three millimeters across. (In a greenhouse, instead of the bird’s eye spot, affected fruits will show pale marbling or a light netting-like pattern.)
Wilting is often the first symptom of bacterial canker in established plants. Foliage of plants with bacterial canker may have dark brown or black lesions that are round or irregular in shape, sometimes with a yellow halo. The pith inside stems might be yellow, reddish brown and mealy (particularly around nodes) or hollow, and stems themselves may split to form light or dark discolored streaks. Cankers can appear around nodes as the infection progresses.
The leaves of plants with bacterial canker sometimes turn yellow, curl, or wilt before eventually darkening to brown and collapsing. Sometimes, just one side of the leaf may show symptoms. Overall growth of plants with bacterial canker can be stunted.
Seedlings do not normally show signs of bacterial canker. When they do, however, leaf margins may show discoloration to brown or black. Contaminated seedlings may wilt or be stunted, and they often die.
What is bacterial canker?
Bacterial canker is a plant disease caused by the bacterium Clavibacter michiganensis sbsp. Michiganensis (CMM or Cmm for short) that affects tomatoes, peppers, and eggplants. Bacterial canker causes lesions on foliage and fruits as well as wilting or curling of leaves and stunted growth. Eventually, plants with bacterial canker may collapse or die. Bacterial canker can spread through a garden quickly, so it’s important for gardeners to know the signs and how to prevent and treat the disease.
The signature symptom of bacterial canker is a raised yellow or brown “bird’s eye spot” on fruit ringed by a white halo. (However, the bird’s eye spot won’t show up if you’re gardening in a greenhouse. Instead, plants with bacterial canker grown in a greenhouse show a marbled or pale net-like pattern on affected fruits.)
The bacteria responsible for bacterial canker spreads in water from rainfall or irrigation, on gardening supplies or the gardener’s skin, or may enter a garden via contaminated seeds or transplants. Bacterial canker especially flourishes in moist, humid weather when temperatures are between 75 and 90 degrees Fahrenheit.
Have you seen bacterial canker on your garden or greenhouse tomatoes? If you have pictures, please send them our way.
Want to learn to more about bacterial canker?
Better Seed covers Bacteria Canker of Tomato
Cornell University covers Bacterial Canker or Tomato
University of Minnesota Extension covers Bacterial Canker of Tomato
Gardening Know How covers Tomatoes with Bacterial Canker
Growing Produce covers Insight on Controlling Bacterial Canker in Tomato
University of California covers Bacterial Canker
Wisconsin Horticulture covers Bacterial Canker
University of Kentucky covers Bacterial Canker of Tomato
Royal Horticulture Society covers Bacterial Canker
Science Direct covers Suppression of Bacterial Canker of Tomato by Composts
The Yardstead covers Bacterial Canker of Tomato found in MN Gardens