Stemphylium leaf blight

Stemphylium leaf blight (Stemphylium vesicarium) is a plant pathogenic fungus. This disease typically attacks leaf tips, purple blotch lesions, and injured or dying onion leaves and is often identified as purple blotch.

Symptoms
Stemphylium vesicarium may enter purple blotch lesions, causing a black fungal growth. Since this fungus is usually found co-infecting with Alternaria porri, symptoms are identical or at least very similar to purple blotch. However, Stemphylium leaf blight lesions appear to be a darker, more olive-brown to black color than do purple blotch lesions. Lesions are often more numerous on the sides of onion leaves facing the prevailing wind. These lesions grow rapidly, coalesce, and cause severe leaf blighting during periods of prolonged leaf wetness.

Control
Practices used to suppress purple blotch generally reduce losses to Stemphylium leaf blight.