How to Manage Pests

UC Pest Management Guidelines

Onion and Garlic

Blue Mold Rot

Pathogens: Penicillium spp.

(Reviewed 1/07, updated 6/08)

In this Guideline:


Blue mold generally appears during harvesting and storage. Initial symptoms include watersoaked 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.


Many species of Penicillium can cause blue mold. These fungi are common saprophytes 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 mycelium grows through the fleshy scales, eventually sporulating profusely on the surface of lesions and wounds. Optimum conditions include moderate temperatures 70° to 77°F (21° to 25°C) and high relative humidity.


Avoid wounds and insect damage to bulbs. Harvest and handle onion bulbs with a minimum of bruising or wounding; most importantly, promptly cure the bulbs so the necks are dry. Store bulbs at temperatures of 41°F (5°C) or less with low relative humidity.


[UC Peer Reviewed]

UC IPM Pest Management Guidelines: Onion and Garlic
UC ANR Publication 3453


R. M. Davis, Plant Pathology, UC Davis
B. J. Aegerter, UC Cooperative Extension, San Joaquin County
T. Turini, UC Cooperative Extension, Fresno County
A. Ferry-Abee, UC Cooperative Extension, Tulare and Kings counties

Acknowledgment for contributions to Diseases:
F. F. Laemmlen, UC Cooperative Extension, Santa Barbara and San Luis Obispo counties
R. E. Voss, Vegetable Crops, UC Davis

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