Downy Mildew (fungus - Bremia lactucae): First symptoms are yellowish or light green areas on the upper surface of the leaves. As the lesions enlarge, the fungus produces downy growth on the underside of the leaf. The affected tissue turns brown, sometimes comprising extensive areas of the leaf. Infected leaves are usually invaded by other microorganisms, causing considerable damage during shipping. The fungus is rarely seed-borne and does not persist long in soil. The source of infection is infested crop residue and, occasionally, weeds. Spores of the downy mildew fungus are wind-borne and are carried considerable distances. Symptoms are produced within 5-10 days after infection. Cool, humid weather with rain, heavy dew or fog favors disease development. There are several races of the fungus and varieties differ in their reaction. Resistant varieties may become susceptible when new races of the fungus develop. Varieties resistant to known races should be planted. Preventative fungicide applications at frequent intervals will give satisfactory control of the disease.
