How to Manage Pests

UC Pest Management Guidelines

SYMPTOMS

Carrot motley dwarf is caused by the combined infection of carrots by two viruses: carrot redleaf virus and carrot mottle virus. Plants infected in the seedling stage will be severely stunted, and yellow to red in color. Plants appear as if they are suffering a nutritional deficiency. When plants are infected at later stages of growth, plants may or may not have stunted growth, but some leaves will be red or yellow, or a combination of red and yellow. Some leaves on infected plants may remain green.

COMMENTS ON THE DISEASE

Carrot motley dwarf primarily occurs in the cooler carrot production areas of California, such as in the Salinas Valley and in the Ventura area, and has been observed in the San Joaquin Valley only in spring carrots. It is not known to occur in the Imperial Valley growing regions. In some years, spring carrots are severely affected. Carrot redleaf virus and carrot mottle virus are transmitted from plant-to-plant only by the willow carrot aphid, Cavariella aegopodii. Both viruses are carried together by the aphid and are then transmitted to plants as the aphid feeds. Once aphids acquire the viruses, they transmit them for the remainder of their lives. Disease development is associated with nearby carrot motley dwarf-affected fields and willow carrot aphids. The host ranges of the carrot motley dwarf viruses are relatively narrow and largely limited to carrot, parsley, and cilantro. The host range of willow carrot aphid is also narrow. Cilantro is not an aphid host, and although it is a virus host, it is not a likely source for subsequent virus spread. Carrots appear to be the best host for both viruses and the aphid vector, and overwintered carrots are the most important inoculum source for subsequent spring carrot motley dwarf development.

MANAGEMENT
Cultural Control
Some resistance, but not immunity, is available in certain carrot cultivars. Typically, Danvers types are most susceptible whereas Imperator types are most resistant. CVC 14 is a good resistant cultivar. Control in coastal areas also can be achieved by avoiding overwintering carrots. As overwintered carrots are the most important source of spring inoculum, attempt should be made to eliminate overwintered fields. If overwintered carrots are present, avoid planting early spring (December-January) carrots within close proximity (about 5 miles).

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

Treatment Decisions
Insecticides are ineffective for control of virus transmission to new crops.

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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