This is a fungal infection that attacks the bulbs of Allium species; onions, shallots, chives and garlic are affected, and to a lesser extent, leeks. The diseases manifests itself as fluffy white mould around the roots, which later forms small black, seed-like storage structures called sclerotia. These spores can remain viable in the soil for at least eight years, and may be activated into germination at any time during this period by a chemical produced by the roots of alliums.

Control

Although traditionally, onions are often grown year after year in the same bed, this can facilitate the build-up of white rot. Crop rotation can help, although the long resting period of spores (8 years of more) may make this impractical. Any affected bulbs should be burned before sclerotia have chance to form. It is also important that spores are not transmitted to other parts of the garden on muddy tools or boots.