How to Manage Pests

UC Pest Management Guidelines

Onion and Garlic

Black Mold

Pathogen: Aspergillus niger

(Reviewed 1/07, updated 1/07)

In this Guideline:


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.


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.


[UC Peer Reviewed]

UC IPM Pest Management Guidelines: Onion and Garlic
UC ANR Publication 3453


R. M. Davis, Plant Pathology, UC Davis
B. J. Aegerter, UC Cooperative Extension, San Joaquin County
T. Turini, UC Cooperative Extension, Fresno County
A. Ferry-Abee, UC Cooperative Extension, Tulare and Kings counties

Acknowledgment for contributions to Diseases:
F. F. Laemmlen, UC Cooperative Extension, Santa Barbara and San Luis Obispo counties
R. E. Voss, Vegetable Crops, UC Davis

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