Cause The alfalfa mosaic virus is carried in tubers, but aphids can transmit it from alfalfa plants to potato plants by feeding first on infected plants and then probing noninfected plants. Nearby alfalfa fields or volunteer infected alfalfa plants in potato fields can be a source of inoculum.
Symptoms Large, irregular, yellow to cream-color spots appear on leaves. Tubers may show internal brown necrotic areas.
Cultural control The disease generally is not a problem that warrants special controls.
- Use certified seed potatoes for seed.
- Control aphids. See the PNW Insect Management Handbook for details.
- Avoid planting potatoes next to alfalfa fields.