The fungus is soilborne and can remain in the field for several years. The fungus first infects dead leaves at the base of the plant and later moves to live tissue. Under moist conditions, infection occurs when plants are reaching maturity. Avoiding infested fields is recommended. Foliage has a general unhealthy yellowish color and plants are small or dwarfed.
Symptoms vary according to type or variety of lettuce.
The virus is seed-borne and overwinters in wild hosts and weeds. Infected seed is the most common source of infection. Once infected plants are present in the field, aphids further spread the virus. The virus can also be mechanically transmitted. There is no cure for mosaic once a plant is infected.