How was your tomato growing season? In many regions, growers were coping with a lot of rainfall, which can enhance many fungal diseases. It also promotes bacterial canker. Clavibacter michiganensis ssp. michiganensis (CMM) is the causative pathogen of this important worldwide disease of tomatoes, which also infects other solanaceous plants, too, including peppers, eggplant and tobacco, and nightshade weeds.

CMM is a seed-borne disease, so prevention begins with purchasing seeds treated and examined to determine if they are pathogen free. Hot water or acid treatment of seeds can control the pathogen, although acid treatment will not destroy pathogens inside the seed, where CMM can be festering. Transplants should be certified to be disease free.

Eliminating the inoculum from fields requires attention, as the bacteria can be transmitted into greenhouses or fields via wind, water, tools, clothing, equipment and stakes. The CMM pathogen can survive for a short time in soil, and up to two or three years in non-decomposed plant debris, while seeds are capable of transmitting the disease even after five years.

“Free moisture is critical for the bacterium to successfully infect the host tissue,” Dr. Hasan Bolkan, former director of Campbell Vegetable Research and Development, said in a webinar on the disease. (See

Warm temperatures and high relative humidity encourage disease spread. Managing the environment and using sanitary practices in the field or greenhouse are key to preventing spread of CMM.

Cultural practices

Bacteria ooze from cankers on the plants, and moisture is needed to transmit them to other plants, where they can enter plant openings or wounds. Rain and wind are primary factors in spreading the disease in the field, as is irrigation or contamination with infected water sources.

Avoid working when plants are wet, including when they are coated with the morning dew. The bacteria can be carried by workers and tools, which readily move the pathogen from plant to plant, and field to field, helping to rapidly spread the disease. If a field is infected, that field should be worked last. All shoes, hands, tools and equipment require disinfection.

Because the pathogen more readily can infest wounded plants, insect damage, pruning injuries and hail damage all increase the disease’s spread. A 10 percent bleach solution with a drop of liquid dish detergent can provide adequate sanitization if left on the surface of tools, equipment or shoes for two minutes. Nonporous surfaces are most readily disinfected and should be used, whereas porous surfaces such as twine or wooden stakes should be discarded after use.

All infected plants should be immediately removed from fields and can be composted in hot piles, where the pathogen will be killed. Plowing plant debris under the field at the end of the season will hasten plant decomposition and decrease pathogen survival.

Recirculated water is another mode of infection. Water must be disinfected before reuse to prevent CMM spread.


Wilting, particularly asymmetrically, is one sign of disease. But it does not confirm bacterial canker, as many other diseases cause plants to wilt. Young plants infected systemically with CMM may die within days, whereas plants that are infected at later stages of maturity die more gradually.

Leaves can also show signs of infection. Tissue necrosis on the leaf margins, called the “firing stage” of the disease, is another primary symptom. On infected plants the oldest leaves hang downward, and the new leaflets curl up. Severe disease can cause total defoliation.

“Mild symptoms on the foliage may easily be overlooked as an injury caused by excess fertilization or by applied pesticides/herbicides or fungal disease symptoms,” Dr. Bolkan said.

Symptoms distinctive to CMM include bird’s eye fruit lesions, which originate as small, white dots. These darken as they grow. On the stem, cankers develop and a cavity can form in the stems. Vascular discoloration, with browning inside the stem, is characteristic. Growers can test for CMM in the field using kits to confirm suspected bacterial canker symptoms.

Some plants may be infected but not show any symptoms for up to seven weeks. Characteristic fruit symptoms may not occur, leading to a false sense of security. Cornell researchers have shown that tomato fruit are most likely to become infected when they are smaller than ½ inch in diameter. So following precautionary sanitary greenhouse and field practices is crucial in disease prevention, even if no signs of disease are present. Studies from Cornell University have shown many strains of bacterial canker exist and have proven that some do overwinter in New York. New strains are being introduced, probably from transplants. (See


Once infected, there is no effective chemical control, although copper sprays at regular intervals following disease detection might minimize spread. Copper fungicides act on the surface of plants only and cannot control pathogens inside the plant. Overuse of copper sprays can damage tissue, however, providing more opportunities for pathogens to enter the plant.

Preventive use of copper sprays, every five to seven days, can provide control if environmental pressures are not too high. However, sprays alone will not be effective in fully controlling the disease, and sanitation protocols must be used in conjunction with copper sprays, too.

Whether in the field or in the greenhouse, bacterial canker of tomatoes and peppers is a concern due to its rapid ability to spread under the right conditions, its ability to live in infected plant tissues and on contaminated surfaces, and the lack of chemical controls or resistant cultivars. Using treated seed, immediately removing diseased transplants and practicing optimal sanitation at all times are the best practices for avoiding crop loss from bacterial canker in tomatoes and other solanaceous crops.