Fusarium basal rot on onions — symptoms and treatment

Severity: high

What is Fusarium basal rot

Fusarium basal rot is caused by Fusarium oxysporum f. sp. cepae. It rots the basal plate — the disc at the bottom of the bulb that the roots grow from — and the roots themselves. Destroy the basal plate and the bulb is severed from its roots: the leaves are still there, but nothing feeds them. Severity is rated high: there is no spray that cures it, the fungus lives in soil, and rot that starts in the field carries on quietly in the store.

Symptoms

Above ground it reads as a plant running out of water. The outer leaves yellow and wilt first, and the crop looks patchy — plants failing here and there rather than in a spreading front, the mark of soil-borne disease. The answer is underground: lift a failing plant and look at the base. You will find pink-white fungal growth, a mat on and around the basal plate, and a soft watery rot of the bulb base: the plate turns brown and watery, the roots rot away, and the bulb lifts free with almost no roots. Plant death follows. Bulbs that look sound at harvest but carry early infection rot in storage.

Key signs:

• Early: outer leaves yellowing and wilting; scattered plants, not a spreading patch
• Early: roots rotting; the plant lifts easily from the soil
• Advanced: pink-white fungal growth at the bulb base
• Advanced: soft watery rot of the basal plate; plant death

Downy mildew and Botrytis leaf blight mark the leaves and leave the base sound. If the leaves are failing and the basal plate is soft and browned, it is basal rot — lifting a plant costs nothing and settles it.

Causes and conditions

This is a soil-borne disease and it behaves like one. The fungus persists in soil for years, with no onion crop needed to keep it going, and infects through the basal plate and roots — especially wounds. That drives most of the damage: injury from cultivation, from careless transplanting, and above all from root-feeding pests such as onion maggot gives the fungus its opening; where those pests are active this disease follows. Warm soil favours it. It moves on anything carrying soil — tools, boots, machinery — and on infected sets. The forma specialis cepae is specialised to onion, which makes long rotation work.

Treatment

There is no curative treatment and no spray that reaches the fungus in the basal plate. Everything effective happens before planting. Destroy affected plants rather than letting them add to the soil population, and keep them out of the store.

Soil solarization and rotation — cultural

Pre-planting. Long crop rotations (5+ years). Solarize soil in summer. Avoid planting in infested fields. Use healthy sets. Rotation of 5 or more years starves out a pathogen specialised to onion — the single most effective measure available. Solarising uses the sun's heat to reduce the fungus in the upper soil. Avoiding known infested ground sounds obvious but matters most, because nothing you apply later makes that ground safe. Healthy sets keep you from importing the fungus yourself.

Prevention

• Rotate for 5 or more years to crops unrelated to onion.
• Plant only clean, healthy sets from a reliable source.
• Control root-feeding pests such as onion maggot — their injuries are the fungus's way in.
• Avoid damaging roots and basal plates during cultivation and transplanting.

Frequently asked questions

Can I save an infected plant? No. The basal plate and roots are already rotting when the leaves start to fail, and nothing applied to soil or foliage restores them. Pull affected plants and destroy them.

Can I store onions from an affected crop? Not with confidence. Infection present at harvest goes on rotting in storage, so sort carefully, use the crop early, and keep suspect bulbs out of the store.

Can I grow onions in that bed next year? No. The fungus persists for years, which is why the rotation is 5+ years. Grow unrelated crops there and put your alliums in clean soil.