How to Manage Pests

UC Pest Management Guidelines

SYMPTOMS

Symptoms on plants infected with carrot thinleaf virus can vary. In general, leaflets appear thinner than normal, giving the plant an overall unusual appearance. Some leaves may have leaflets that are distorted and show a mosaic pattern. Leaflets of plants infected at a young stage may be extremely thin, hence the name of the virus. The virus has not been shown to cause yield losses in California.

COMMENTS ON THE DISEASE

Carrot thinleaf virus is only transmitted from plant to plant by aphid vectors. Aphids transmit the viruses during the probing phase of aphid visitation to plants; transmission does not occur during aphid feeding. Aphids only retain the ability to transmit these viruses for very short periods of time (minutes to a few hours). Thus, spread is often very rapid and occurs in local areas of a field. Many aphid species can spread carrot thinleaf potyvirus. Field spread of carrot thinleaf potyvirus, and potyviruses in general, occurs when aphid activity in fields is high.

Carrot thinleaf virus has a very limited plant host range; in nature it seems to be largely limited to carrots. In California, carrot thinleaf virus is primarily found in the Central Valley, mostly in the lower San Joaquin Valley carrot production area. Some fields have been found to have a significant incidence of carrot thinleaf, but have not suffered economic loss. However, in Washington State, where it sometimes coinfects plants with other carrot viruses, disease losses can result. Carrot thinleaf virus oversummers in volunteer carrots that survive from previous carrot plantings.

MANAGEMENT
Cultural Control
No cultivar resistance is known. A likely cultural control strategy would be to eliminate volunteer carrots during the late summer as newly planted seedlings are emerging; aphid activity is high at this time, and the virus can be spread to new fields.

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

Treatment Decisions
There are no effective pesticide strategies. Insecticides directed at controlling the aphid vectors are ineffective because they cannot kill the aphids before transmission occurs.

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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