How to Manage Pests
UC Pest Management Guidelines
Bacterial Leaf Spot
Pathogen: Xanthomonas campestris pv. vitians
(Reviewed 4/17, updated 4/17)
In this Guideline:
Symptoms AND SIGNS
An early symptom of bacterial leaf spot is 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 characteristic of this disease). If the 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.
Comments on the Disease
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. It has also been found growing epiphytically on weed plants, but the significance of this in disease development in lettuce is not known.
To prevent bacterial leaf spot:
Organically Acceptable Methods
Use cultural controls in an organically certified crop.
Copper fungicides can be used, but are not very effective; they must be applied before infection occurs.
UC IPM Pest Management Guidelines: Lettuce
S. T. Koike, UC Cooperative Extension, Monterey County
Acknowledgment for contributions to Diseases:R. M. Davis, Plant Pathology, UC Davis