🧅 Onion How hard it is to control: ★★★★☆ 4/5

Fusarium basal rot on onion

Fusarium oxysporum f. sp. cepae enters through the base plate and roots, rotting the bulb from below. It develops in warm soil and often only shows up in store.

Symptom photos

Fusarium basal rot — Onion cut in half — rotted basal plate and scales browning from the base — Onion cut in half — rotted basal plate and scales browning from the base Photo: Howard F. Schwartz, Colorado State University, Bugwood.org, licence CC BY 3.0

Symptoms

Leaves yellowing and drying from the tips
Brown, dry rot of the base plate
White-pink fungal growth on the base plate and between the scales
Roots dying back and falling away
Bulbs soft around the base plate, but with no foul smell

What it does to the crop

Plants collapsing in the field in the second half of the season
Bulbs rotting in store from the base plate up
A weaker crop and small bulbs
The fungus persisting in the soil for years

What encourages it

Warm soil above 25°C
Damage to the base plate by onion fly and nematodes
Alliums grown after alliums
Wet, poorly aerated soils

Prevention

Rotate with a minimum 4-year break
Plant healthy sets and treat the planting material
Control onion fly, which damages the base plate
Avoid physical damage during routine work
Lift and dry the onions promptly

⚠️ Before you reach for a spray

This atlas is informational and still growing — different diseases can look almost identical. Confirm the diagnosis and read the product label before applying any plant protection product. If you are unsure, ask your local extension or advisory service.

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