This manual covers fungal and Oomycete disease of lettuce. The extent of losses caused by these diseases to lettuce crops varies considerably according to the disease type, method of crop production, lettuce variety, season and other factors. Many lettuce diseases can cause considerable damage and occasionally complete crop loss. Some disease types such as downy and powdery mildews can spread very quickly to affect most plants in crop. Others such as sclerotinia and ring spot tend to remain localised. A few pathogens, notably Fusarium oxysporum and Sclerotinia spp., can persist in soil for many years.