How to Manage Pests

UC Pest Management Guidelines

Lettuce

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.

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)
Calculate impact of pesticide on air quality
Bee precaution pesticide ratings
When choosing a pesticide, consider its usefulness in an IPM program by reviewing the pesticide's properties, efficacy, application timing, and information relating to resistance management, honey bees (PDF), and environmental impact. Not all registered pesticides are listed. 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.

[Precautions]

PUBLICATION

[UC Peer Reviewed]

UC IPM Pest Management Guidelines: Lettuce
UC ANR Publication 3450

Diseases

S. T. Koike, UC Cooperative Extension, Monterey County
T. Turini, UC Cooperative Extension, Fresno County

Acknowledgment for contributions to Diseases:
R. M. Davis, Plant Pathology, UC Davis

Top of page


Statewide IPM Program, Agriculture and Natural Resources, University of California
All contents copyright © 2017 The Regents of the University of California. All rights reserved.

For noncommercial purposes only, any Web site may link directly to this page. FOR ALL OTHER USES or more information, read Legal Notices. Unfortunately, we cannot provide individual solutions to specific pest problems. See our Home page, or in the U.S., contact your local Cooperative Extension office for assistance.

Agriculture and Natural Resources, University of California

Accessibility   /PMG/r441100111.html revised: June 15, 2017. Contact webmaster.