Onion white rot

What it is

Onion white rot is one of the most feared allotment diseases because there is no treatment and the fungus survives in the soil for many years, possibly longer than fifteen. Once a bed is infected, it is effectively closed to the onion family for a long time.

Symptoms

Leaves yellow and wilt from the tips, usually starting with a few plants in a patch that then spreads along the row. Affected plants come out of the ground easily because the roots have rotted. The base of the bulb is covered with a fluffy white fungal growth, and soon after you will see tiny, round black bodies like poppy seeds embedded in it. Onion fly also causes collapse, but you will find maggots rather than white fluff.

Cause

The black specks are sclerotia, resting bodies that lie dormant in the soil until they sense chemicals given off by allium roots. They then germinate and infect the plant. Disease is worst in cool, moist soil in spring and early summer. Sclerotia are moved around on boots, tools, wheelbarrows, in soil on bought plants, and in infected sets or garlic cloves.

Control

Dig up affected plants as soon as you notice them, together with the soil around their roots, and put them in the household waste, never on the compost heap. Clean tools and boots before moving to another bed. Harvest the healthy rest of the crop and use it promptly. No fungicide is available to home gardeners for this disease.

Prevention

Buy sets and garlic from reliable suppliers and never plant cloves from shop-bought bulbs. Keep a long rotation, and on an infected plot grow alliums in raised beds or containers filled with fresh compost. Avoid sharing soil or plants from infected ground, and treat any new allotment with care until you know its history.

Plants often affected

• Onions
• Garlic
• Leeks
• Chives
• Ornamental onion

Could it be something else?

These problems share some of the same symptoms. Compare them, or run the symptom checker with these signs ticked.

• Clubroot
• Honey fungus
• Onion fly
• Carrot fly

Months to keep an eye out: April, May, June, July and August.

Sources

Identification and life-cycle details were checked against the references below. Text and advice are our own. See our editorial policy.

• Wikipedia — Sclerotium cepivorum
• UC IPM — White rot of onion and garlic