Clavibacter michiganensis subsp. michiganensis
The first symptom is the downward turning and wilting of the lower leaves of the plant. Leaves may exhibit unilateral wilting and light colored streaks may extend up and down the outside of the leaf midrib, petiole and stem. These streaks may break open to form cankers. Infected leaves and petioles characteristically remain attached to the stem. Internally, the stems show light brown or yellow vascular discoloration, which progresses to reddish brown, and often the pith turns yellow, becomes mealy and hollow. A yellow bacterial ooze can be squeezed from the cut end of an infected stem. Fruit infection occurs as small, white lesions, which develop into brown, scabby lesions. These are surrounded by white halos, giving the lesions a birds-eye appearance. Typically, the vascular tissue extending from the stem scar into the fruit will have a yellow-brown discoloration, and cavities may develop in the pith. These fruit symptoms are common in the greenhouse.
Conditions for Disease Development
Infection generally occurs through wounds in the plant tissue, however, it can also occur through the leaf stomata or roots. The bacterium can survive for up to five years in the soil and infected plant debris. It can also survive on weeds, volunteer tomato plants and seed. Secondary spread occurs from splashing water, contaminated equipment and tools used in pruning, clipping and transplanting operations. Moderate (18-24°C, 65-75°F) temperatures and greater than 80% relative humidity favor disease development. Optimum moisture conditions for plant growth, low light intensity and high nutrient concentrations, especially nitrogen, also enhance disease development. Symptoms tend to be more severe in sandy soils than in organic soils.
The use of clean seed and transplants, as well as sterilization of the transplanting mix, flats and all equipment used in the transplanting operations can reduce losses from this disease. Transplants should not be topped or mowed since secondary spread can occur rapidly this way. A rotation to a non-host crop for at least three years can reduce losses from bacterial canker.