How to Manage Pests

UC Pest Management Guidelines

SYMPTOMS

Bacterial leaf blight is often first noticed in fields as brown areas about 3 to 4 feet in diameter. Leaf symptoms appear as irregular brown spots, often beginning on the leaf margins. Lesions initially have an irregular, yellow halo and may appear watersoaked. Spots coalesce and cause a leaf blight and dark brown streaks develop on leaf petioles. Floral parts may also be blighted. A sticky amber-colored bacterial exudate, which is a diagnostic sign of the disease, may be present on leaves or observed flowing downward on petioles and flower stalks.

COMMENTS ON THE DISEASE

Xanthomonas campestris pv. carotae is seedborne and survives on and is spread with carrot seed. The bacteria also survives in carrot debris but cannot survive in the soil in the absence of debris. Rain or sprinkler irrigation is required for optimum disease development. Warm weather favors infection and disease development. Optimum temperatures are between 77°F (25°C) and 86°F (30°C); infection does not occur below 65°F (18°C). The pathogen is dispersed in splashing water. Plant-to-plant spread may occur under heavy dew conditions.

In most carrot-growing areas bacterial blight does not warrant control. In a few areas, such as the Antelope Valley, severe outbreaks may occur.

MANAGEMENT

Cultural Control
Plant Xanthomonas-indexed seed or treat seed in a hot water dip. Use furrow or drip irrigation rather than sprinklers. Turn under carrot residue to hasten decomposition. Avoid continuous carrot culture by using a 2- to 3-year crop rotation scheme.

Organically Acceptable Methods
Cultural controls, hot water dips, and sprays of certain copper sulfate formulations are acceptable for use on organically grown produce.

Monitoring and Treatment Decisions
Assay seed and treat if pathogen is found. Foliar treatments occasionally necessary if rainy weather persists.

PUBLICATION

UC IPM Pest Management Guidelines: Carrot
UC ANR Publication 3438
Diseases
R. M. Davis, Plant Pathology, UC Davis
F. F. Laemmlen, UC Cooperative Extension, Santa Barbara Co.
J. Nunez, UC Cooperative Extension, Kern Co.
T. A. Turini, UC Cooperative Extension, Imperial Co.
Acknowledgment for contributions to the diseases section:
B. W. Falk, Plant Pathology, UC Davis

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