Why Is There White Fluffy Mould on My Onion?

Dense white fluffy mould growing at the base of onion stems, just where they meet the soil, sometimes with tiny black pinhead-sized bodies (sclerotia) visible in the white growth — this is onion white rot, caused by Sclerotinia cepivorum. It is one of the most serious diseases in the vegetable garden, not because it is dramatic or fast-spreading within a season, but because the sclerotia it produces persist in the soil for 20 years or longer, making the affected ground essentially permanently hostile to alliums unless managed with great care. If you have identified white rot in your garden, understanding what it means for your long-term growing plans is essential.

How white rot works

Sclerotinia cepivorum germinates from its sclerotia in the soil only in response to specific volatile chemicals released by the roots of allium plants — garlic, onion, leek, chive, and shallot. This is why rotating away from alliums is the primary management strategy: no allium roots, no germination trigger, no active fungus. Once germinated, the fungus grows through the soil to infect the nearest allium root or bulb base. It spreads slowly from plant to plant within a bed, usually appearing as a cluster of affected plants rather than a uniform infection across the whole bed. The white fluffy growth is the sporulating mycelium; the embedded black sclerotia are the long-term survival structures.

Immediate response

Remove all affected plants — foliage, bulbs, and as much of the surrounding root zone soil as you can practically remove — and dispose of them off-site. Do not compost them. Do not allow soil from the affected area to be moved to other parts of the garden on tools, boots, or wheelbarrows, as sclerotia are easily transported. Mark the affected area clearly. Ideally, do not grow any allium in that area for as many years as possible — the longer the gap, the lower the sclerotial germination rate when you eventually return alliums to that ground.

Can white rot-affected ground be treated?

There is no practical chemical treatment available to home gardeners. In commercial production, various fumigants and bio-fungicide treatments (Trichoderma-based products) are used, but these are not widely available for garden use in the UK. The "garlic water drench" technique — applying diluted garlic juice to the soil to trigger mass sclerotial germination in the absence of a host crop, thereby depleting the bank — is discussed in some organic growing literature, but the evidence for its effectiveness in garden-scale application is limited. The most reliable approach remains strict long-term rotation.

Growing alliums in a white rot garden

If your garden is small and total avoidance is impractical, growing alliums in containers with fresh bought-in compost completely avoids the soil-borne risk. Container growing will never replace open-ground growing for large crops, but for a household's garlic or a small onion harvest, it is a viable workaround while the affected bed sits idle for as many years as possible.