Powdery mildew of lettuce is one of the most common diseases of lettuce and occurs everywhere lettuce is produced. The disease is characterized by a white powdery appearance on the upper leaf surfaces and the wrapper leaves in crisphead lettuce. In advanced infections, both the upper and lower leaf surfaces are affected and the leaves can take on a deformed and buckled appearance. The disease will rarely kill the lettuce plant but because of the cosmetic damage, the head will be unmarketable. The disease is very a common problem in greenhouse production, when ambient moisture may be higher and more constant than that in field production. Under such conditions, if robust management programs are not in place, a disease epidemic can quickly damage an entire production run.