Certified GKH Gardening Expert
Based on my research, it appears that chemical applications are recommended to apply to lettuce beds before the on-set of bottom rot. It is recommended to remove old plant debris after every growing season (since the pathogen can survive between crops) and to solarize the soil in late spring or summer. Bottom rot exists in wet growing conditions so you should look at growing a resistant cultivars of lettuce in well drained soil or in a raised bed where they are spaced enough apart for good air circulation.
If only the outer leaves are affected, you can harvest the lettuce after removing the outer leaves. However, if the fungus has invaded the entire head or the lesions are too deep, the plant should be discarded.
For more information about lettuce diseases, please visit the following link: