Bacterial leaf spot of lettuce

Early symptoms of bacterial leaf spot are small (less than 0.25 inch in diameter), water-soaked leaf spots on the older leaves of the plant. These lesions are typically bordered by leaf veins and angular in shape. Lesions quickly turn black—a diagnostic character of this disease. If disease is severe, numerous lesions may coalesce, resulting in the collapse of the leaf. Older lesions dry up and become papery in texture, but retain the black color. Lesions rarely develop on newly developing leaves.

Bacterial leaf spot can occur on both leaf and head lettuce varieties. As with most bacterial diseases, the pathogen, Xanthomonas campestris pv. vitians, is highly dependent on wet, cool conditions for infection and disease development. Symptoms develop only if rain or sprinkler irrigation is present. Splashing water from these sources moves the bacteria from plant to plant. The pathogen is seedborne, though research indicates that commercial seed used in California is relatively free of the pathogen. In the case of lettuce seedlings grown as transplants, the pathogen may become established on plants during the greenhouse phase of growth. The bacterium can also survive on undecomposed lettuce residue and be spread to subsequent lettuce crops. It has also been found growing epiphytically on weed plants, but the significance of this in disease development in lettuce is not known.