Lettuce Downy Mildew
Lettuce Downy Mildew (Bremia lactucae) like Vine Downy Mildew belong to the class of primitive fungi known as Oomycetes.
It attacks any stage of lettuce with stmptoms first appearinf as chlorotic yellow spots on the upper leaf surface. Under damp cool conditions combined with moisture on the leaves a white fungus develops on the lower leaves. The spores produced by these mycelium go onto infect other parts of the plant.
In time these lesions turn a brown and die, sometimes affecting the whole leaf. Older leaves tend to get attacked first.