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How to Manage Pests

UC Pest Management Guidelines

| More pests | More crops | About guidelines |

Lettuce

Bacterial Leaf Spot

Pathogen: Xanthomonas campestris pv. vitians

(Reviewed 8/07, updated 8/07)

In this Guideline:

SYMPTOMS

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.

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. 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.

MANAGEMENT

The use of pathogen-free seed is the first step in disease management. However, reliable seed assays and established threshold levels are not yet available. When possible, avoid sprinkler irrigation. Avoid planting back-to-back lettuce crops if the first crop was severely diseased and infected lettuce residue is present.

Organically Acceptable Methods
Cultural controls are acceptable for use on 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/Acre R.E.I.+ P.H.I.+
(trade name)   (hours) (days)
When choosing a pesticide, consider the general properties of the fungicide as well as information relating to environmental quality.
 
A. COPPER HYDROXIDE Label rates 24 0
  MODE OF ACTION GROUP NAME (NUMBER1): Multi-site contact (M1)
 
 
+  Restricted entry interval (R.E.I.) is the number of hours (unless otherwise noted) from treatment until the treated area can be safely entered without protective clothing. Preharvest interval (P.H.I.) 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 (for more information, see http://www.frac.info/). 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 fungicide with a different mode of action Group number.

[Precautions]

PUBLICATION

[UC Peer Reviewed]

UC IPM Pest Management Guidelines: Lettuce
UC ANR Publication 3450
Diseases
S. T. Koike, UC Cooperative Extension, Monterey Co.
R. M. Davis, Plant Pathology, UC Davis

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Statewide IPM Program, Agriculture and Natural Resources, University of California
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