The epidemiology of lettuce mosaic was studied on two farms in Buckinghamshire during 1947–50. Two patterns of spread were obvious on Farm A during 1947–8: (1) spread within a crop from sources initiated by seed-borne virus; and (2) spread into a crop from nearby infected old plants. The first type caused serious losses only when aphids were unusually numerous and the incidence of seed-borne infection was high; serious losses more often resulted from the second type. Winter crops usually suffered severely, and they initiated a cycle of infection in the spring which continued from crop to crop throughout the year when aphids were sufficiently numerous. Spread from crop to crop was much reduced during 1949–50 by planting or drilling lettuce in large blocks, well separated from each other, and by omitting a winter crop. Killing the infesting aphids with an insecticide did not prevent the introduction of virus from nearby diseased crops. The insecticidal treatment of old diseased and aphid-infested crops (but before alatae are produced) is recommended.
More diseased plants often occurred near hedges, woods, trees or buildings than in the open field, presumably because such barriers halted flying aphids.
Three species of aphids were common on lettuce: Nasonovia ribis-nigri (Mosley), which infests the heart leaves; and Macrosiphum euphorbiae (Thomas) and Myzus persicae (Sulzer), which infest the outer leaves. M. euphorbiae and M. persicae are vectors of lettuce mosaic, but the former is less effective than M. persicae which, although rarely as numerous as the other two species, is the most important vector. All three species overwintered on lettuce, and migrated from the winter crops during April–May.
One obvious need to control lettuce mosaic is the production of virus-free seed. Without this, however, the disease can be ameliorated by adopting these measures whenever practicable: lettuce crops should be planted or drilled in large blocks, well separated from each other; diseased crops and crop-remains should be destroyed as soon as possible; winter crops should be treated with an insecticide in late November, and insecticides should be used to prevent the production of large aphid populations, especially on old crops.