Advice
RHS Help & Advice
Onion white rot
A fungal disease that causes the bulbs of Allium species to rot, devastating crops.
Hosts
Allium species, especially onions, garlic, shallots and chives. Less severe on leeks.
Symptoms
Leaves yellow and may wilt and the plants are loose in the soil. The disease rots the roots and attacks the bulb, causing a white fluffy mould which later forms small (0.5mm diameter) black, seed-like resting structures called sclerotia.
Biology
The sclerotia can remain alive but dormant in the soil for at least eight years (although some sources suggest it can be as much as 15 years). Chemicals produced by the roots of alliums stimulate them to germinate and the fungal mycelium then attacks the roots, spreading to the bulb. The disease is not airborne and is transmitted principally in contaminated soil. It is most severe in cool, wet summers.
Control
Destroy plants promptly before sclerotia form. Do not compost. Be careful to remove all affected parts, to prevent sclerotia from contaminating the soil. Avoid spreading contaminated soil on boots or tools. The long survival in soil makes rotation impractical in most cases. There are no chemical treatments available to gardeners.