Onion Purple Blotch

Alternaria porri

Another member of the notorious Alternaria gang, this is one of the most common garlic, onion and leek diseases. It likes warm (25 degrees C), wet weather and first manifests itself as small, purplish, watery lesions on the leaves. These eventually enlarge and turn brown (and may girdle and kill the leaf). There may also be a yellow halo around the lesion.  

The best way to deal with this fungus is to keep your plants well fed and watered, as healthy plants are rarely seriously affected. In ideal warm, wet conditions it produces wind borne spores and can spread to neighboring plants, so if any plants show signs of infection, remove and destroy them. The spores overwinter on crop debris so clean up the beds at the end of the summer, to reduce sources of infection for the following year. Keep the soil well drained and provide good air circulation. The feeding of thrips may make plants more vulnerable to purple blotch disease.

Image: Howard F. Schwartz, Colorado State University, Bugwood.org