Symptoms and Signs
Comments on the Disease
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.