Bacterial canker of tomato has been diagnosed recently in the Plant Disease Clinic. The disease is caused by the bacterium, Clavibacter michiganensis subsp. michiganensis. The disease can be devastating to tomato production, however, the disease appears sporadically from year to year and may be more severe in dry years.
Early symptoms of the disease usually begin on lower leaves. Margins of leaflets become necrotic and often the leaflets along one side of a leaf wilt. Eventually, entire leaves and branches wilt and die. As the bacterium moves into the vascular tissue of plants, yellow streaks may develop along stems of affected plants and occasionally, stem cankers are formed. The vascular tissue inside the stems becomes yellow and later turns reddish-brown. The pith of infected plants may completely collapse. A yellow ooze may be seen on the cut ends of systemically infected stems when squeezed. Leaves and branches in the upper portions of infected plants wilt and die as the bacteria moves up through the stems.
Fruit symptoms begin as small, raised lesions surrounded by a white margin or halo. The lesions expand to 2-4 mm in diameter and the centers becomes dark brown. Lesions are often referred to as birds-eye spots. Often, several lesions are formed where fruits touch in a cluster, giving the fruits a scabby appearance. Stems of fruits may also have a or color. Fruits produced late in the season on severely affected plants are severely stunted, misshapen, and mottled.
The most likely source of bacterial canker is from contaminated seed and transplants. However, the bacterium survives in overwintered tomato debris, Solanaceous weeds, volunteer plants, wooden stakes, and contaminated seed. The bacteria can be spread from plant to plant by splashing water, worker's hands, and on contaminated equipment. The bacterium survives and multiplies on the plant surfaces, causing little disease. Wounds and abrasions from pruning, staking, and possibly wind-driven rain and hail provide entry for the bacteria into the plant tissue. Once the bacteria are introduced into the vascular tissue, the disease progresses rapidly. Plants infected early may die and set no fruit. Plants infected late in the season may show little reduction in yield.
Clean seed and transplants are the most effective control measures. Crop rotation with a nonhost plant is recommended. Remove infected debris and sterilize wooden stakes in a 1% bleach solution. Avoid wetting the foliage during irrigation. Pruning instruments should be disinfected between cuts. Avoid wounding plants. Tolerance to bacterial canker is present ina few processing-tomato cultivars, but not in fresh-market tomato cultivars.
This article originally appeared in the July 29, 1994 issue, pp. 1994 issue, pp. 125-126.