Symptoms and Signs
Initial symptoms include water-soaked or pale-yellow areas on the outer surface of scales. Later, a green to blue-green, powdery mold may develop on the surface of the lesions. Infected areas of fleshy scales are tan or gray when cut. In advanced stages, infected bulbs may disintegrate into a watery rot, often because of secondary infection by bacteria or yeasts.
Comments on the Disease
Many species of Penicillium can cause blue mold. These fungi are common on plant debris and senescent plant tissue. The pathogens usually invade onion bulbs and garlic through wounds, bruises, or uncured neck tissue. Once inside the bulb, the mycelium grows through the fleshy scales, eventually sporulating profusely on the surface of lesions and wounds. For onions, secondary infections by other pathogens after an initial blue mold infection are more serious than blue mold alone.
Blue mold generally appears during harvest and storage. Germination of fungal spores is optimal at 70° to 77°F. High relative humidity favors the growth of the fungus.
Management
To reduce the risk of blue mold, promptly cure the bulbs so the necks are dry. Additionally, use the following other cultural practices:
- Minimize bruising or wounding bulbs during harvest and handling, as well as from insect damage.
- Store bulbs at a maximum temperature of 41°F and at low relative humidity.