A white fungal growth on the base of onion bulbs is a sign of the disease white rot. This fungus affects onions, and leeks, but is less common on chives, garlic and shallots.
The leaves of the onion become yellow, die back prematurely and the plants may fall over as the rot progresses. The roots of the affected plant are usually found to be rotten and the base of the bulb is invariably covered with a white fungal growth. At a later stage, numerous small, black sperical resting bodies will appear and as they develop, the bulb may become completely rotten. In this country the disease is usually most troublesome during warm summers or warm spells in autumn and spring in the case of overwintering onions.
Infected onions must be lifted and destroyed by burning and if possible the soil around the bulb should be carefully removed and also burnt. Never leave infected plants in the ground or add them to the compost heap. Take care to avoid spreading infected soil on tools and foot wear.
The disease can remain in the soil and so it important not to grow onions in the same patch for at least eight years.