Onion Neck Rot

Botrytis species

This fungus attacks onion and shallots in late summer, entering through the neck (especially if damaged) and causing it to go soft and brown and develop gray fuzzy growth. Commonly the rot appears on infected bulbs once they are in storage. To make sure this doesn’t happen avoid damaging the tops (which can allow spores to enter the neck) and cure them properly.

The spores overwinter on crop debris, so keep the garden beds clear. Rotate all of your Alliums regularly. Use certified seed or sets (white varieties are more susceptible than yellow or red). Avoid over-fertilizing with too much nitrogen as this can delay maturation. Keep the plants well watered while young, but stop watering when the bulbs stop swelling, so they can mature properly. Too much late rain can increase problems with this disease. Also make sure you the bulbs are fully cured before storing them. If you can identify infected plants discard them, so they don’t rot in storage.

Image: Howard F. Schwartz, Colorado State University, Bugwood.org