Onion Smudge

This fungal disease attacks maturing bulbs and is visible as concentric spots of black or dark green mold. It occurs in warm, (75-85 degree F) wet conditions.

The usual precautions taken against fungal diseases apply here. Make sure the soil is well drained and has good air circulation and plenty of compost. Remove and destroy infected plants promptly. Clean up crop debris at the end of the growing season, to reduce sources of infection for the following year.

Some onion varieties are resistant (Early Yellow Globe).

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