How to Manage Pests

Pests in Gardens and Landscapes

Blue mold rot on garlic and onions—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

Many species of Penicillium can cause blue mold. The fungi are common on plant debris and senescent plant tissue. Invasion of onion bulbs and garlic is usually through wounds, bruises, or uncured neck tissue. Once inside the bulb, the fungus grows through the fleshy scales, eventually sporulating profusely on the surface of lesions and wounds. Optimum conditions include moderate temperatures and high relative humidity.


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.

Browning of garlic bulbs and yellowing of foliage
Browning of garlic bulbs and yellowing of foliage

Blue mold rot on onions
Blue mold rot on onions

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