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.
Management
Cultural Control
To prevent bacterial leaf spot:
- Use pathogen-free seed as the first step in disease management. However, reliable seed assays and established threshold levels are not yet available.
- Avoid sprinkler irrigation when possible.
- Avoid planting back-to-back lettuce crops if the first crop was diseased and infected lettuce residue is present because the bacterium can survive on undecomposed lettuce residue and be spread to subsequent lettuce crops.
Organically Acceptable Methods
Use cultural controls in an organically certified crop.
Chemical Control
Copper fungicides can be used, but are not very effective; they must be applied before infection occurs.
| Common name | Amount per acre | REI‡ | PHI‡ | |
|---|---|---|---|---|
| (Example trade name) | (hours) | (days) | ||
| Not all registered pesticides are listed. The following are ranked with the pesticides having the greatest IPM value listed first—the most effective and least likely to cause resistance are at the top of the table. When choosing a pesticide, consider information relating to the pesticide's properties and application timing, honey bees, and environmental impact. Always read the label of the product being used. | ||||
| A. | COPPER HYDROXIDE | |||
| (various products) | Label rates | see label | 0 | |
| MODE-OF-ACTION GROUP NAME (NUMBER1): Multi-site contact (M1) | ||||
| ‡ | Restricted entry interval (REI) is the number of hours (unless otherwise noted) from treatment until the treated area can be safely entered without protective clothing. Preharvest interval (PHI) is the number of days from treatment to harvest. In some cases the REI exceeds the PHI. The longer of two intervals is the minimum time that must elapse before harvest. |
| 1 | Group numbers are assigned by the Fungicide Resistance Action Committee (FRAC) according to different modes of actions. Fungicides with a different group number are suitable to alternate in a resistance management program. For fungicides with mode-of-action group numbers 1, 4, 9, 11, or 17, make no more than one application before rotating to a fungicide with a different mode-of-action group number; for fungicides with other group numbers, make no more than two consecutive applications before rotating to a fungicide with a different mode-of-action group number. |