Alfalfa Mosaic Virus
- Many ornamentals and weeds
Alfalfa mosaic virus (AMV) is in the genus Alfamovirus and is spread by aphids.
Yellow mosaic or calico patterns occur on the foliage. Some strains of AMV can cause severe stunting of plants and tuber necrosis. Sometimes corky or brown areas will develop in potato tubers. Foliage develops bright yellow blotches interspersed with green, called “mottling.” Leaves can curl and also develop a bronze discoloration. On alfalfa and some other crops, the phloem tissue is killed, resulting in plant death. This symptom has not been documented on hemp. In addition, the effects of this virus on cannabis yield and flower quality have not been determined.
The virus is commonly found in alfalfa. Aphids feed on infected alfalfa, acquire the virus, and transmit it to healthy potato plants. Aphids need to feed on another infected plant to spread the disease again. The virus is not seedborne in potatoes. Alfalfa mosaic virus is harbored in many virus-infected alfalfa fields and weeds. AMV is spread by several species of aphids, and it is unknown if Cannabis aphid is a vector. Aphids acquire the virus from an infected plant and transmit it to hemp by probing leaf tissues. Aphids with the virus are only able to transmit it for a few minutes to a few hours. Infections usually occur in hemp fields grown near alfalfa, and only on plants closest to the alfalfa.
Time for Concern: Throughout the growing season.
When and Where to Scout: Starting in late spring, observe plants for unusual leaf coloration, especially on plants growing closest to alfalfa fields.
Threat Level: Low.
- Insecticides to control aphids will be of limited help.
- The best management option is to avoid planting potatoes close to alfalfa fields.
- Avoid planting hemp close to alfalfa, if possible.
- Remove plants with positive diagnosis.