Purple blotch is a fungal disease caused by Alternaria porri that primarily affects onions, garlic, leeks and other alliums. Small, water-soaked lesions enlarge into distinctive purple to brown oval spots, often surrounded by yellow tissue. Older lesions may develop concentric rings.

The fungus thrives during warm, humid or wet weather. Spores spread by wind and splashing water and can survive on infected plant debris. As lesions enlarge and merge, leaves may collapse, reducing the plant's ability to produce and mature a good-sized bulb.

How to Fix It

Remove badly infected leaves and dispose of infected plant debris at the end of the season. Rotate alliums with unrelated crops, and start with healthy bulbs, sets or transplants.

Give plants plenty of airflow and keep foliage dry whenever possible. Water at soil level rather than overhead, avoid overcrowding, control weeds around the crop and make sure the soil drains well.

Biological fungicides containing Bacillus species may provide some preventive suppression, but they won't cure infected tissue. For the home garden, good sanitation, dry foliage and crop rotation are the most useful controls.