Onion White Rot: How to Recognise and Manage It

White rot is a serious soil-borne fungus that attacks the onion family — onions, garlic, leeks, shallots and spring onions. There is no cure and no spray you can use to save an infected crop. Once it is in your soil, the job is no longer about treatment; it is about recognising it early and stopping it from spreading any further.

That sounds bleak, and it is fair to be honest about it. But white rot is also slow-moving and stays put — it travels on contaminated soil, not on the wind. With careful hygiene and a long break from growing onions in that spot, most gardeners can keep cropping elsewhere on the plot for years. This guide is part of our cluster on growing onions; start there for the full sowing-to-harvest method.

How to recognise it

White rot tends to show itself from late spring through summer, often when the bulbs are swelling and you are starting to look forward to a harvest. The classic sequence is:

• Yellowing, wilting leaves. The foliage goes yellow and floppy from the tips down, and the plants look like they are dying back far too early. A single plant or a small patch is affected first.
• Plants pull up easily. Give a wilting plant a gentle tug and it lifts with almost no resistance, because the roots have rotted away.
• Fluffy white fungus at the base. Around the base of the bulb and over the roots you will see a dense, white, cotton-wool-like growth. This is the giveaway — it is far more solid and fluffy than ordinary surface mould.
• Tiny black dots (sclerotia). Embedded in that white fluff are pinhead-sized black specks, like ground pepper. These are the fungus's resting bodies, and they are what makes white rot so persistent.
• Soft, rotting bulbs. The bulb itself turns soft and rots. There is nothing to save — affected bulbs cannot be eaten or stored.

If you see yellowing leaves but no white fluff and no black dots at the base, it may be something far less serious — simple overwatering, poor drainage, or onions starting to bolt in a hot spell. White rot is confirmed only when you find that white growth and the black sclerotia on the roots. It is easy to confuse a struggling onion with a diseased one, so always lift a plant and check the base before you panic.

Why it persists

The reason white rot is so feared is those black sclerotia. They are the fungus in a dormant, armoured state, and they can survive in soil for a very long time — commonly quoted as 7 to 15 years, with no allium crop needed to keep them alive. They simply wait.

When you next plant onions or garlic in that ground, chemicals released by the growing roots wake the sclerotia, they germinate, and the cycle starts again. A handful of black specks left in the soil this year can ruin a crop more than a decade from now.

Crucially, white rot does not spread on the wind. It moves the way soil moves:

• on the soil clinging to your boots and wellies,
• on tools — forks, trowels, dibbers — used in an infected bed,
• on contaminated soil you move around the garden, including in barrows and on the soles of shoes,
• occasionally on infected sets, bulbs or plants bought in or swapped with other gardeners.

Because it travels on soil rather than air, your own habits decide whether it stays in one bed or ends up across the whole plot. That is the good news hiding inside the bad: containment genuinely works.

What to do

If you find white rot, act calmly but firmly to remove the source and avoid spreading those sclerotia.

1. Lift and bin every affected plant — bulb, roots and surrounding soil. Carefully dig out the diseased plants together with a generous trowelful of the soil around the roots, where loose sclerotia will be sitting.
2. Do not compost any of it. A home compost heap never gets hot enough to kill sclerotia, so composting infected material just breeds the problem and then spreads it everywhere you use the compost. Put it in your household waste, not the green-waste bin and not the compost.
3. Bag it on the spot. Drop the lifted plants and soil straight into a bag rather than carrying them open across the garden, so you are not scattering infected crumbs of soil as you walk.
4. Clean your tools and boots before going anywhere else. Knock off all soil and wash your fork, trowel and the soles of your boots, ideally away from your other beds. This single step does more than anything to stop white rot reaching clean ground.
5. Do not move that soil around. Don't dig the bed over and barrow the soil elsewhere, and don't use it to fill pots or top-dress other beds.

How to manage and prevent it

You cannot eradicate white rot from infected soil, but you can keep growing onions and garlic successfully elsewhere, and you can avoid bringing it in at all.

Keep alliums off infected ground for years

Once a bed has had white rot, treat it as off-limits to the whole onion family — onions, garlic, leeks, shallots and spring onions — for as long as you reasonably can, ideally the full 7 to 15 years. A standard three- or four-year rotation is nowhere near long enough for white rot. The bed is perfectly fine for other crops in the meantime: beans, brassicas, roots, salads and so on are unaffected.

A crop rotation planner helps you keep track of which beds have grown which families and when, so an infected bed doesn't quietly come back into the allium rotation by accident.

Grow in containers or fresh soil

If you only have one plot and white rot has spread, growing onions and garlic in containers of fresh, bought compost sidesteps the problem entirely — the fungus isn't in clean compost. Use generous pots or troughs, peat-free multipurpose compost, and never reuse soil from an infected bed. This is also a tidy way to keep growing your favourite varieties, such as the reliable maincrop onion 'Sturon' or autumn-planting garlic like 'Solent Wight', while the infected ground sits out its long sentence.

Buy clean planting material

Prevention is far easier than cure, so avoid importing the disease in the first place:

• Buy onion sets and garlic from reputable suppliers rather than swapping bulbs of unknown history with other gardeners. Our guide to growing onions from sets vs seed covers choosing healthy planting stock.
• Growing from seed is the lowest-risk route of all, because seed cannot carry white rot the way a contaminated set or bulb can.
• Inspect bulbs before planting. Discard any sets or cloves that already look soft or have suspicious white growth around the base.

Practise good plot hygiene

Make tool and boot cleaning a normal habit, not just an emergency response. Knock soil off tools between beds, keep a stiff brush by the shed, and be especially careful if you have visited an allotment or another garden where white rot might be present — the soil on your boots is the most likely way it arrives. These same habits protect against other soil-borne problems too.

White rot is one of the few growing problems where honesty matters more than reassurance: there is no quick fix, and pretending otherwise just spreads it. But it is also slow, local and beatable through patience and clean habits. Keep your tools clean, give infected ground a long rest, lean on containers and fresh compost, and you can carry on enjoying home-grown onions and garlic for years. When you are ready to plant again on clean ground, head back to the main onion growing guide for the full method.