Anthracnose disease may show up soon on coastal California lettuce. For the second spring in a row, rainy weather is creating conditions necessary for lettuce anthracnose, caused by the fungus Microdochium panattonianum, to develop on the very earliest crops. Initial symptoms are small, water-soaked spots on outer leaves. Spots enlarge, turn orange tan, and later will be covered with the white to pink fungal growth 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.
The fungus persists in the soil as resilient microsclerotia that allow it to survive for long periods of time in the ground. These sclerotia can 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 and other field professionals are advised to keep records of where the disease occurs; 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.