In California, lettuce anthracnose disease rarely develops without spring rains. The rains in March and April have created conditions that resulted in some anthracnose outbreaks in the coastal region. Initial symptoms are small, water-soaked spots on outer leaves. Spots enlarge, turn orange tan in color (Photo 1), and later will be covered with the white to pink fungal growth (Photo 2) of the pathogen. If disease is severe, the lesions can coalesce and cause significant dieback of the leaf and in some cases can result in stunting of the plant. In advanced stages, the center tissues of the spots can dry and fall out, resulting in a “shot-hole” symptom (Photo 3).
The fungus persists in the soil as resilient survival structures (microsclerotia), which produce spores when conditions are favorable for the pathogen. Falling rain water splashes the microsclerotia and spores from soil onto leaves, resulting in infection. Anthracnose of lettuce is an unusual disease in which a soilborne pathogen causes only a foliar disease of the plant host. Because this is a soilborne pathogen, growers are advised to keep records of where the disease occurs, as this information will help predict outbreaks in future wet springs. Anthracnose is controlled well with protectant fungicides, though such materials must be applied prior to infection.
PHOTO 1. Early symptoms of lettuce anthracnose consist of orange tan spots.
PHOTO 2. Anthracnose is most readily recognized when the fungus produces its white to tan spots.
PHOTO 3. In advanced stages, anthracnose causes leaf tissue to fall out, creating a “shot- hole” effect.