Onion white rot

Stromatinia cepivora

also called allium white rot

A soil fungus of the onion family: leaves yellow and wilt, and the roots and base rot under a fluffy white mold studded with tiny black grains.

Fungus No cure Soil-borneSpread on tools & hands Spreads to neighbors

What you'll see

A fluffy white mold coats the base of the bulb and the roots, dotted with pinhead-sized hard black grains.
The leaves yellow, wilt and topple, often in a spreading patch through the row.
The roots rot away so the plant lifts out easily, and the bulb base turns soft and rotten.
Bulbs already rotting in store were infected in the ground before lifting.

Why it happens

A soil-borne fungus that attacks the roots and base of alliums. It survives in the soil as tiny resting bodies, called sclerotia, that are triggered into growth by chemicals leaking from allium roots.

Cool, moist soil in spring and early summer favors it; the resting bodies can lie dormant in the ground for fifteen years or more, waiting for an allium crop.

Treating it

There is no cure; lift and destroy affected plants together with the soil clinging to the roots.
Do not compost infected material, and do not shake the soil off onto clean ground.
Harvest any sound-looking bulbs from an infected bed promptly and use them, as they will not store.
Accept the loss of that crop and turn your attention to keeping the fungus from spreading.

Keeping it away

Never grow alliums on infected ground again — the resting bodies persist for many years.
Clean soil off boots, tools and barrows so you do not carry the fungus to clean beds.
Do not move soil, or plants raised in it, from an infected plot.
Raise onions, leeks and garlic from seed or clean sets rather than importing soil-grown transplants.
Keep a strict record of any affected bed and grow alliums elsewhere.

Who gets it

All the onion family — bulb onions, salad onions, garlic, leeks, shallots and chives.