Alfalfa (lucerne) mosaic

Potato plant with 'calico' disease caused by alfalfa mosaic virus.

Cause

Alfalfa mosaic virus (alfamovirus).

Symptoms

Symptoms can vary depending on the host, but nearly always involve a yellow-green mosaic pattern or bright yellow blotches on leaves. As the plant matures, these areas may become more intense in colour and cover large areas of the leaves.

How it spreads

Aphids spread the virus from plant to plant, needing only very short feeding times for transmission. Many aphid species are able to transmit the virus, including

• green peach aphid (Myzus persicae)
• pea aphid (Acyrthosiphon pisum)
• bluegreen aphid (Acyrthosiphon kondoi)
• spotted alfalfa aphid (Therioaphis trifolii).

Annual crops are most likely to become infected when planted near perennial hosts such as lucerne and clover that often have a high level of infection.

Crops affected

Alfalfa mosaic virusoccurs worldwide and has a wide range of hosts among legumes and other broad-leafed plants.

Natural hosts of the virus include:

• lucerne
• clover
• potato (calico disease)
• tomato, capsicum
• lettuce, soybean
• bean
• sowthistle (Sonchus oleraceus)
• nightshades (Solanum).

Control

The level of alfalfa mosaic infection in vegetable crops is usually not sufficient to cause serious losses. Adequate control is possible by not planting susceptible crops adjacent to stands of lucerne and clover.

Chemical registrations and permits

Check the Australian Pesticides and Veterinary Medicines Authority database for chemicals registered or approved under permit to treat this pest on the target crop in your location. Always read the label and observe withholding periods.