How to Manage Pests
UC Pest Management Guidelines
Also known as shot hole or ring spot, anthracnose initially causes small (less than 0.125 inch or 3 mm), water-soaked spots on outer leaves. Spots enlarge, turn yellow, and are usually irregular and angular in shape. Under cool, moist conditions, white to pink spore masses of the fungus will be visible in the center of the lesions. If disease is severe, the lesions will coalesce and cause significant dieback of the leaf and in some cases result in stunting of the plant. As spots age, the affected tissue will dry up and become papery in texture. Eventually the centers of these spots will fall out, resulting in the shot hole symptom.
Anthracnose lesions are often clustered along the midribs of lower leaves. Romaine cultivars, in particular, exhibit severe disease along leaf midribs. If infected early and severely, young lettuce seedlings can be killed by anthracnose.
This disease requires cool, wet conditions for infection and symptom development. Anthracnose is always associated with rainy springs. Splashing water moves microsclerotia onto lettuce leaves, resulting in infection. The fungus, Microdochium panattonianum, is host specific to lettuce and can survive for up to 4 years as microscopic resting structures (microsclerotia) in the soil. Romaine cultivars are particularly susceptible.
To prevent disease development, avoid planting early spring lettuce in fields having a history of the disease. Rotate with any crop other than lettuce to help reduce soil inoculum levels, though such rotations will not eliminate the pathogen unless lettuce is not planted for over four years. Use irrigation systems (furrow or drip irrigation) that eliminate leaf wetting. Resistant cultivars are not widely available.
Organically Acceptable Methods
UC IPM Pest Management Guidelines: Lettuce