Basal rot / white rot (onion & garlic)
Soil-borne rots of the base plate and roots — browning and softening (basal rot) or white fluffy mould with black sclerotia (white rot) — in field and store.
- Pathogen
- Fungi (Fusarium / Sclerotium cepivorum)
- Spreads by
- Soil-borne; infected sets/seed; sclerotia
- Favoured by
- Infested soil; poor drainage; continuous onion
- Main risk
- Base & roots rot; bulbs fail in field & store
Overview
Basal rot and white rot are soil-borne diseases that attack the base plate (the short stem at the bottom of the bulb) and roots of onion and garlic, in the field and in storage. Fusarium basal rot (Fusarium oxysporum) causes the leaves to yellow and die back from the tips, the roots to rot, and the base plate to brown and soften, often with a light pink-white mould; the bulb rots from the base up and does not keep. White rot (Sclerotium cepivorum) is even more damaging: it rots the base and roots, covers them with a white, fluffy mould, and studs them with numerous tiny black sclerotia (like poppy/mustard seeds).
In both, the above-ground plant yellows, wilts and can be lifted easily because the roots have rotted, and losses continue in storage. White rot is especially serious because its sclerotia survive in the soil for many years, so an infested field stays infested. Both are favoured by cool, moist conditions (white rot) or warm soils (basal rot) and by continuous onion/garlic. Because there is no cure once the base is rotted, management rests on clean, disease-free seed/sets, long rotation, good drainage, roguing and careful curing and storage.
How to identify it
- Leaves yellowing and dying back from the tips, plants wilting and lifting easily because the roots have rotted.
- Basal rot: browned, softened base plate with a light pink-white mould, bulb rotting from the base up.
- White rot: white, fluffy mould on the base and roots studded with numerous tiny black sclerotia (like poppy seeds).
- Losses continuing in storage as affected bulbs rot and fail to keep.
What causes it and when
- Soil-borne fungi — Fusarium oxysporum (basal rot) and Sclerotium cepivorum (white rot) — attacking the base plate and roots.
- White rot sclerotia survive in the soil for many years, so an infested field stays infested.
- Spread through infested soil, and on infected sets, bulbs and seed.
- Basal rot favoured by warm soils, white rot by cool, moist soils; both worse with continuous onion/garlic and poor drainage.
Cultural & organic control
- Plant only clean, disease-free sets/bulbs and treated seed, and grow tolerant varieties where available.
- Practise long rotation away from onion/garlic (especially against white rot), and ensure good drainage.
- Rogue out and remove affected plants with the soil/sclerotia around them, and destroy infected debris.
- Cure and dry bulbs well before storage and add bio-agents (e.g. Trichoderma) to the soil where recommended.
Chemical control
- Treat sets/seed with a recommended fungicide before planting to reduce early infection.
- A soil/root-zone application of a recommended fungicide or bio-agent can help in infested fields, but cannot cure rotted bulbs.
- Combine with rotation, drainage, curing and sanitation — chemicals alone will not clear sclerotia-loaded soil.
- Confirm the current seed/set treatment and soil measure for your crop with your local KVK.
Prevent it coming back
- Make clean sets/seed, long rotation and good drainage the routine, especially in white-rot fields.
- Rogue affected plants with their soil, destroy debris, and cure and dry bulbs before storing cool and dry.
- Avoid carrying infested soil, sets or bulbs into clean fields, and build soil bio-agents.
Crops most at risk
Where this problem is called out in our crop guides — open a crop for the full package of practices.
Frequently asked questions
My onions/garlic are yellowing, lifting easily and rotting at the base with white mould and tiny black balls. What is it?
Rotted roots that let the plant lift easily, a rotting base, and white fluffy mould studded with tiny black sclerotia point to white rot (Sclerotium cepivorum); a browned, softened base plate with a pinkish mould and no black balls is Fusarium basal rot. Both rot the bulb from the base and continue in store. There is no cure once rotted — use clean sets/seed, long rotation, good drainage, rogue affected plants, and cure bulbs well before storage.
What are the tiny black balls on the base of my rotting garlic/onion?
Those tiny black balls, about the size of poppy or mustard seeds, on the white mould at the base and roots are the sclerotia of the white rot fungus (Sclerotium cepivorum). They are how the fungus survives in the soil for many years, so seeing them confirms white rot and warns that the field is now infested for a long time. That is why long rotation, clean sets and strict sanitation — not just a spray — are essential.
Why does white rot make a field unusable for onion for years?
Because the white rot fungus forms tiny sclerotia that survive in the soil for many years even without a host, so once a field is infested it stays a threat to onion and garlic for a long time, and each new crop revives the fungus. That long soil survival is why white rot demands long rotation away from onions, strictly clean sets and seed, roguing affected plants with their soil, and preventing infested soil from being carried into clean fields.
Symptoms and control measures follow ICAR, KVK and state agricultural-university package-of-practices; any named chemical is a registered use. Always read the product label and confirm the dose for your crop and zone. editorial policy.