Severely affected plants are inedible, because the fungi have invaded their circulatory systems, causing them to rot from within. As the disease progresses, plants flop over and die.
Make sure lettuce plants get good air circulation and plenty of sun, and keep weeds controlled to promote prompt drying of the area after rains. Grow slow-maturing heading varieties at wide spacing in raised rows or beds, and mulch them to reduce splashing of mud onto the leaves. Avoid using sprinklers or other overhead irrigation methods after lettuce has formed firm hearts or heads. Most gardeners do best by making several small plantings of lettuce in spring and late summer, which creates a more diverse garden, which in turns limits problems with this and many other garden diseases. In beds where this disease has been seen, grow grains and other non-susceptible crops for three years.
Pull up affected plants and compost them. Do not turn under diseased plants, because they may rot so slowly that infected plant material may still be present in the soil in spring. Take steps to improve drainage in the affected bed, because moldy lettuce is a common symptom of inadequate drainage.