How to Manage Pests

Pests in Gardens and Landscapes

Blue mold rot—Penicillium sp.

Blue mold rot generally appears during harvesting and storage. Initial symptoms include water-soaked 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.

Life cycle


Avoid wounds and insect damage to bulbs. Handle onion bulbs with a minimum of bruising or wounding; dry the bulbs with prompt curing of neck tissue. Store bulbs at temperatures of 41° F or less with low relative humidity.

Blue mold rot on onions
Blue mold rot on onions

Statewide IPM Program, Agriculture and Natural Resources, University of California
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