Black mold of onion is caused by the fungus Aspergillus niger. This disease can occur in the field or during storage. As the common name suggests, the primary symptom is a black discoloration of tissue. Infected bulbs may show blackening at the neck, streaks or spots of black on or beneath the outer scales, and black discoloration in bruised areas. In advanced stages, the entire bulb appears black and shriveled.
The fungus Aspergillus niger is common in the soil and on dead plant material. Infection of bulbs usually occurs through wounds (especially through the neck as onions mature and tops break over or are cut). The fungus may also enter through wounds on the roots or bruised outer scale tissue. Control of any foliar diseases will help reduce black mold during storage. It is also important to avoid wounding when bulbs are harvested, transported, or stored. Finally, proper drying and storage will help prevent the occurrence of black mold. Warmth and moisture favor disease development. For successful storage of onions, thorough ventilation, a low temperature , dry atmosphere, and protection against freezing are essential.
For information on recommended storage conditions for various vegetables, refer to Pm-731, "Harvesting and storing vegetables".
Crop rotation, good soil drainage, and the use of clean seed or healthy transplants will help prevent the occurrence of bulb diseases in the field.
This article originally appeared in the March 15, 1996 issue, p. 30.