"Lettuce drop prefers cool moist conditions. The disease affects plants when they reach maturity and comes on suddenly casing the lettuce head to wilt and die. Under the dying plant is cottony white mycelium with hard small black sclerotia that look like mouse turds. It is caused by two fungii: S. minor which infects under drier conditions and S. sclerotiorum. Both diseases can survive in the soil as sclerotia for 2 - 3 years without a host Alternate hosts include: canola, potatoes, beans, and many weeds. The fungus infects the plant though dead tissue during conditions of free water on leaves. "
Possibly?