Which of the following symptoms is most commonly associated with downy mildew of lettuce?

The symptom most commonly associated with downy mildew of lettuce is the presence of white/purple sporulation. Downy mildew is caused by the pathogen Peronospora, and it characteristically leads to the development of a downy, powdery mold on the undersides of leaves. This sporulation often appears white or purplish, depending on environmental factors and the specific strain of the pathogen.

This symptom is crucial for identification and diagnosis of downy mildew, as it distinguishes this disease from others that may impact lettuce. The visual appearance of this sporulation often occurs in humid conditions, which is conducive to the development and spread of the pathogen.

The other symptoms listed, while they may occur with different plant diseases, are not specifically associated with downy mildew in lettuce. Blotchy canes and brown web scarring do not relate to the typical symptoms exhibited by downy mildew. Chlorotic spots could occur in various diseases and stress conditions but are not the hallmark symptom of downy mildew. Recognizing the specific symptom of white/purple sporulation enables proper identification and treatment of downy mildew in affected crops.