How to Manage Pests

UC Pest Management Guidelines

SYMPTOMS

Early symptoms of corky root are yellow bands on tap and lateral roots of lettuce seedlings. These yellow areas gradually expand, taking on a green-brown color and developing cracks and rough areas on the surface of the root. As disease severity increases, the entire taproot may become brown, severely cracked, and nonfunctional; the feeder root system will also be reduced and damaged. At this point, roots are very brittle and easily break off when examined. Corky root may cause internal discoloration of the root. When the root is severely diseased, aboveground symptoms consist of wilting during warm temperatures, stunting of plants, and general poor and uneven growth. Corky root symptoms could be confused with ammonium toxicity, which causes a brick-red discoloration of the central portion of the root and wilting of lettuce foliage.

COMMENTS ON THE DISEASE

The corky root bacterium, Rhizomonas suberifaciens, is a soilborne pathogen that is prevalent in most coastal lettuce growing areas but may not be present in inland regions. Corky root affects both leaf and head lettuce varieties. Disease is typically more severe when soil temperatures are warmer. Corky root is worse in fields where lettuce is grown consecutively. High soil nitrate levels can increase disease severity.

MANAGEMENT

Rotate crops out of lettuce; do not grow lettuce consecutively. Avoid over fertilizing with nitrogen fertilizers. Some corky root resistant cultivars are now available. For corky root infected crops, growers may need to add additional fertilizer and water in order to bring the crop to maturity. High, well-draining beds may sometimes reduce corky root severity.

PUBLICATION

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

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