Onion - White Rot
The disease rots the roots and attacks the bulb, the leaves wilt, turn yellow, and in some cases the plants appear loose in the soil.
This usually manifests itself from mid-summer until early autumn, particularly if the weather has been cool and wet.
On closer inspection one can see a white fluffy mould which later forms small black, seed-like structures called sclerotia cepivorum.
The attack is generally less severe on leeks.
The sclerotia can remain alive but dormant in the soil for at least 8 years, and are encouraged out of dormancy by chemicals produced by plants of the allium family.
These stimulated sclerotia germinate and produce a fungal mycelium that attacks the roots, eventually spreading to the bulb.
The disease is not airborne like some diseases, it is transmitted principally in contaminated soil, and is most severe in cool, wet summers.
There are no chemical treatments available to amateur gardeners.
Destroy affected plants immediately, do not compost them, and ensure that you remove all affected parts to prevent sclerotia from contaminating the soil.
Even if you use a Crop Rotation plan this will not always be successful due to the longevity of dormancy period.