Cause Alfalfa mosaic virus (AMV), which is transmitted by aphids, in seed, through pollen, and mechanically. The pea aphid, Acyrthosiphon pisum, is the main vector, but many other aphid species can transmit the virus.
Symptoms Symptoms, most apparent during cool spring and autumn conditions, are variable because there are many strains of AMV. Alfalfa varietal genetics and weather conditions affect symptom expression. Symptoms range from a mild yellow and green mottle to a bright yellow mottle to necrosis and death of the plant. There can also be dwarfing and leaf distortion in some alfalfa varieties. Infected plants are predisposed to drought or freeze injury.
Cultural control Although aphid-resistant varieties may help minimize localized spread, no practical control measures are available at present.