Lettuce leaves with brown spots are not appealing to eat. Frequently the inner leaves of infected plants escape damage.
Choose resistant varieties when they are available. Make sure plants get good air circulation and plenty of sun, and keep weeds controlled to promote prompt drying after rains. Use mulch to keep soil from splashing onto plants during heavy rains. Avoid using sprinklers or other overhead irrigation methods after lettuce heads or hearts have formed.
Clip off and compost badly affected leaves. With the return of dry weather, leaf-type lettuce plants will usually produce a new crop of sound leaves from the plants' centers.