Bacterial canker of tomato

Bacterial canker of tomato is a serious tomato disease caused by ''C. michiganensis ssp. michiganensis''. The disease was first discovered in 1909 in Grand Rapids, Michigan, USA but is currently reported in tomato production areas worldwide and outbreaks occur annually. However, preventive measures can be taken at all stages of production to avoid losses from bacterial canker.

Seedlings
Marginal necrosis, tan to dark necrotic patches on the leaves and stems, and small white raised blisters on infected leaves may be symptoms of bacterial canker infection on young plants. Stunting, wilting and stem splitting can also occur, especially in grafted seedlings. However, symptoms can take several weeks to develop following infection and therefore may not be visible at the seedling or transplant stage.

Leaf and plant
Leaf yellowing and necrosis around leaf margins called “firing” or “marginal necrosis” can indicate a foliar and/or systemic infection. When the stems or petioles are cut open, discoloration of the vascular tissues may be seen. In greenhouse-grown plants, symptoms appear as interveinal chlorotic to pale green patches that quickly become necrotic, giving a scorched appearance. Infected plants wilt beginning with the lower, older leaves, or leaves above the point of infection. Wilting may be asymmetric, appearing more on one side of the plant than the other. Infected leaves die, and light brown streaks or cankers, which may darken with age, develop on infected stems. Typical cankers can be common in the field but are rarely seen in the greenhouse. The vascular tissues become light brown to reddish brown and the pith appears mealy, brown and dry. Older plants tend to be less susceptible than younger ones and the disease tends to be more severe on plants infected early vs. late in their growth cycle.

Fruit
Small dark spots on the fruit surrounded by a white halo or “bird’s-eye” spots are characteristic of bacterial canker on field-grown fruit. Spots become raised and the centers turn brown with age. Infections, and the resulting spots, occur when bacteria are deposited on fruit by splashing water from rain or overhead irrigation, or mechanically during handling of the plants. When internally infected fruit are opened, yellowing or browning caused by the decay of the tissues may be seen. In the greenhouse, bird’s-eye spots are typically not observed, but fruit may appear netted or marbled, or they may remain symptomless.

Prevention
Only purchase seed that has been tested and found to have no evidence of ''C. m. ssp. michiganensis''. : Use only clean trays and flats for transplants. Use overhead watering sparingly – provide only enough moisture for seedling growth. Allow foliage to dry before the sun sets, as prolonged leaf wetness can lead to increased disease development, and use other strategies to minimize time of leaf wetness. Inspect plants frequently for canker and other disease symptoms. Greenhouses should be kept clean and thoroughly disinfected between crops. Hands and tools should be sanitized between plants or rows with a disinfectant solution to minimize pathogen spread.