Symptoms and Signs
Carrot motley dwarf of cilantro and parsley is caused by the combined infection by two viruses: Carrot redleaf virus and Carrot mottle virus. Plants infected in the seedling stage will be severely stunted and the foliage can be yellow, orange, or red in color. Plants appear as if they are suffering a nutritional deficiency. When infected at later stages of growth, plants may or may not have stunted growth, but some leaves will have the yellow, orange, or red discoloration. Some leaves on infected plants may remain green.
Comments on the Disease
Carrot motley dwarf primarily occurs in the cooler cilantro and parsley 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 cilantro. It is not known to occur in the Imperial Valley growing regions. In some years, spring cilantro is severely affected. This virus complex also causes symptoms on carrot and dill.
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 cilantro, parsley, or carrot fields and willow-carrot aphids.
The host range of willow-carrot aphid is narrow. Cilantro is not a preferred host of this aphid species, and although it is a virus host, it is not likely a significant 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.
Keep the willow-carrot aphid vector under control. Remove weed reservoirs from around fields and plant cilantro in fields without a history of this disease.
Organically Acceptable Methods
Use cultural controls in an organically certified crop.
Manage the vector, willow-carrot aphid, with insecticides, though such sprays do not prevent the aphids from transmitting the virus.