Today 
The common, quick-spreading fungal disease Botrytis cinerea strikes in damp, humid weather, and often enters the lettuce through a cut or tear in a leaf. Affected leaves are inedible and should be thrown away but not composted. The worst scenario is when the lettuce is attacked at the base and a slimy, yellowish orange-brown rot forms, which can the kill the entire plant.
Solution
Organic
Promptly remove any infected parts of the lettuce. Try to reduce the risk of damage to lettuce leaves by drowning slugs and snails in beer traps, or go on a night-time patrol and remove them by hand. Space plants out to ensure good air circulation, and water early enough in the day to allow the leaves to dry out before nightfall. Also remove any garden debris that might harbour the fungus.
Chemical
Although there aren't any specific chemicals available to treat grey mould, sulphur dust will incidentally reduce the problem.
