A white fungal growth on the base of onion bulbs is a sign of the disease called 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 spherical resting bodies will appear and as they develop, the bulb may become completely rotten.

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 footwear.

The disease can remain in the soil and so it is important not to grow onions in the same patch for at least eight years.