All types of lettuce (head, leaf, romaine/cos) are susceptible to downy mildew, and plants are susceptible at all growth stages. Seedlings may die from severe infections, but established plants are rarely killed. Localized lesions usually develop on lower, older leaves first. The lesions are initially light green to yellow on the upper leaf surfaces and often have an angular shape because larger veins limit their growth. With time, the lesions turn brown and become necrotic. One to two weeks after infection, mats of white, fluffy, fungal-like growth develop on the undersides of leaves. This white growth includes spores that spread the pathogen from plant to plant. On rare occasions, downy mildew infections can become systemic within the plant and cause brown discoloration of internal stem and root tissues. Infected leaf tissue can continue to decay during transportation and storage. The necrotic tissues provide a site of infection for secondary pathogens that can result in soft-rotting resulting in further damage.