How to Manage Pests

UC Pest Management Guidelines

SYMPTOMS

Black rot starts in the carrot crown at the point of leaf attachment, beginning with the outermost petioles; eventually all the leaves can be killed. The upper part of the carrot root will show a dark rot, which occasionally may continue into the lower part of the carrot root. Leaves weakened or rotted by black rot may break off when gripped by mechanical harvesters, resulting in roots being left in the ground. Black rot can also be a problem in storage.

Under certain conditions, the fungus also can cause a leaf blight, which is characterized by dark brown lesions along the leaf margins. The fungus can also cause a pre- and postemergence damping-off of carrot seedlings.

COMMENTS ON THE DISEASE

The fungus is favored by sprinkler irrigation or rainy weather and high temperatures, which may predispose tissue to infection. Alternaria radicina is seedborne and may be spread on carrot seed. It also survives in carrot debris and in the soil for several years. Black rot is important only in certain areas or regions in California. It is common in coastal mountain valleys, e.g., the Cuyama Valley.

MANAGEMENT

Cultural Control
Plant Alternaria-indexed seed or treat seed in a hot water bath; this is especially important in fields where black rot has not been identified or carrots have not been grown.

Furrow rather than sprinkler irrigation may reduce disease development. Crop rotation is highly recommended to prevent buildup of the fungus in the soil. Deep tillage may provide some control by burying inoculum of the fungus away from the carrot crown.

Organically Acceptable Methods
Cultural controls are acceptable for use on organically grown produce.

Monitoring and Treatment Decisions
Assay seed; if pathogen is found, treat seed. Foliar applications of fungicides to control black rot are marginally effective.

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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