These lesions are typically bordered by leaf veins and angular in shape. 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. As with most bacterial diseases, the pathogen, Xanthomonas campestris pv.
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. In the case of lettuce seedlings grown as transplants, the pathogen may become established on plants during the greenhouse phase of growth. It can also survive on undecomposed lettuce residue and be spread to subsequent lettuce crops. The bacterium has also been found growing epiphytically on weed plants, but the significance of this in disease development in lettuce is not known.