Black Mold

The fungus is first detected as an area covered with a black sooty mass (made up of microscopic spores) on the neck or outer scales, where injury, disease, or toppling of the foliage has caused an opening in the skin. Fungal spores may develop between dry, dead outer scales and the first inner fleshy scales of the bulb. Invaded scales initially become water-soaked, and spores often appear in a streaking pattern along veins. Occasionally, interior parts of the bulb may be affected with gray-to-black discoloration without showing external symptoms. Under dry conditions, diseased scales dry and shrivel, and black masses of spores are visible between outer scales. Affected parts may also be invaded by soft rot bacteria, causing the whole bulb to deteriorate into a watery soft rot.

Black mold is primarily a postharvest condition. It occurs on both onions and garlic, but is more of a concern in onion crops. Black mold occurs most commonly where onions or garlic are grown under warm conditions, such as the desert areas of California. Moisture must be present on the bulb for 6 to 12 hours for the disease to develop. This can occur if bulbs are damp when stored, or when they are going into and coming out of storage if condensation accumulates on the bulbs between the outer dry scales.

The fungus survives on decaying organic matter such as plant debris, and is a weak pathogen that generally does not affect uninjured bulbs. The fungus is widely distributed among soil environments.

While some fungicide control programs used for foliar diseases may reduce the incidence of black mold (with the exception of many fungicides applied to control the pathogen that causes downy mildew), there are no fungicides for the direct control of black mold. Culturally manage black mold by doing the following:

• Store and transport bulbs at temperatures below 59°F and at low humidity to slow growth of the fungus.
• Reduce bruising and injury during harvest, handling, and transport to minimize the opening of invasion sites for the fungus.
• Harvest onions promptly and do not delay drying. Do not use heated air for drying.
• Maintain stable temperatures during transport, as well as when bulbs are going into and coming out of storage.
  • Wide temperature fluctuations can cause condensation on the bulbs and between the outer dry scales, which promotes the development of black mold.