Agriculture: Onion and Garlic Pest Management Guidelines

Black Mold

Symptoms and Signs

Black mold occurs on both onions and garlic. The fungus is first evident at the top or sides of the bulb where disease or injury has caused an opening in the skin. The fungus develops between dry, dead outer scales and the first inner fleshy scales of the bulb. Invaded scales initially become water soaked. Under dry conditions diseased scales dry and shrivel, and black masses of spores are visible between outer scales. Diseased scales may also be invaded by soft rot bacteria, causing the whole bulb to deteriorate into a watery soft rot.

Comments on the Disease

Black mold occurs most commonly where onions or garlic are grown under warm dry conditions such as the desert areas of California. It is more of a concern in onion crops than in garlic. The fungus survives on decaying organic matter such as plant debris.


There are no chemicals for the direct control of black mold. Research indicates that a good fungicide control program for foliage diseases will reduce the incidence of black mold. Storage and transit temperatures below 55°F (12.8°C) and as low as 33°F (0.6°C) are recommended to suppress black mold development. Handling of bulbs to avoid bruising also reduces injury and invasion sites for the fungus.

Text Updated: 01/07