White rot on garlic — symptoms and treatment

Severity: high

What is White rot

White rot is caused by Sclerotium cepivorum. It can persist in soil for 20+ years and is devastating for allium crops — which is why it is the most serious disease a garlic grower can face. There is no cure and no spray. Once the fungus is in a piece of ground, that ground is effectively lost to garlic, onions and leeks for a generation.

Symptoms

Above ground the first sign is easy to misread: yellowing and wilting of the older leaves, starting with the outermost and working inward. It often shows up in a patch — a cluster of plants going backwards while their neighbours look fine. Affected plants pull out of the ground far too easily, because the fungus has destroyed the roots and basal plate.

The diagnosis is at the bulb base. Lift a failing plant and look: white fluffy mycelium coats the base of the bulb and the roots. Embedded in that fungal mat are small black sclerotia — hard, round bodies scattered through the white growth like ground pepper. That pairing of white fluff plus tiny black specks is conclusive.

• Early: yellowing and wilting of older leaves; a patch of failing plants; plants lifting out easily
• Diagnostic: white fluffy mycelium on the bulb base and roots; small black sclerotia in the fungal mat
• Advanced: bulb rot; plant death

Causes and conditions

Sclerotium cepivorum survives in soil as sclerotia — the black bodies you see in the fungal mat. They sit dormant and viable for many years, germinating only when they sense the chemicals that allium roots release into the soil. Without a host they simply wait, and each infected plant returns a fresh crop of them to the soil, so one bad season restocks the field.

Cool, moist soil favours the disease. It does not blow in on the wind — it arrives in things you carry: infested soil on boots, tools and machinery, soil clinging to transplants, runoff water, and, most commonly of all, infected planting stock. A single contaminated batch of seed garlic can infest clean ground permanently.

Treatment

There is no chemical control and no rescuing an infected plant. Management is prevention, exclusion and containment.

Long rotation and clean seed — cultural

Timing: pre-planting. Rotate away from alliums for 15+ years in infested fields. Use certified disease-free seed. Do not compost infected material.

In infested fields, rotating away from alliums for 15 or more years denies the sclerotia the host they need. Certified disease-free seed keeps the fungus out of clean ground in the first place — infected stock is the main way White rot travels. Never compost infected material: compost will not reliably kill sclerotia. Bag and dispose of infected plants instead.

Prevention

• Plant only certified disease-free seed garlic — never save or share stock from a bed with any history of White rot.
• Keep infested ground out of allium rotation for 15+ years; treat the whole area as compromised, not just the visible patch.
• Clean soil off boots, tools and machinery before moving from a suspect bed to a clean one, and work the suspect patch last.
• Pull and bag failing plants with the root ball intact, and never compost infected plants or their soil.

Frequently asked questions

Can I treat White rot with a fungicide? No. Once plants show white mycelium and black sclerotia at the bulb base, the crop is lost and the priority shifts to stopping the spread.

How long before I can grow garlic there again? The fungus can persist in soil for 20+ years, and the recommended rotation for infested fields is 15+ years away from alliums. Plan on that ground being unavailable for garlic, onions and leeks for a very long time — and all alliums must come out of the rotation, not just garlic.