Cabbage Root Fly
Risk near you, next 48h
See whether your local forecast favours this disease right now.
When it strikes
Fix the cause, not the leaf
No spray will help here. The trigger is soil, water supply, nutrition or temperature, so change the growing condition and judge the result on new growth, not on damaged leaves.
Soil and crop debris
It waits in the ground or in last season's debris, so removing the plant does not clear the bed. Rotation and debris removal are the working controls.
It carries over to next season in soil, debris or seed — which is why rotation matters here, not just treatment.
Safe to compost
No pathogen carries over on this material, so it can go on an ordinary heap with the rest of the garden waste.
When and where to look
Scout the planting while temperatures hold between 10°C and 25°C.
Rule these out first
Same category, and at least one host crop in common — so they are genuinely confusable in the same part of the garden.
The signs that lead here
Full problem lists for these crops
About
Cabbage root fly (Delia radicum) lays eggs at the base of brassica stems, and the larvae eat the fine roots away. Transplants are far more vulnerable than direct-sown plants because their root systems are small and already disturbed.
Symptoms
Young plants wilt in sunshine, recover overnight, then wilt worse each day until they die. Leaves take on a blue-grey cast. Pull a failing plant and the roots are ragged stumps riddled with white maggots.
Organic Treatment
Fit a brassica collar — a 12 cm disc of felt or cardboard fitted snugly around the stem at soil level — on every transplant. The fly lays on the soil surface next to the stem and the collar removes that surface entirely.
Chemical Treatment
No garden insecticide is effective once larvae are in the roots, and preventive soil treatments are no longer available for home use. Physical exclusion is the control.
Prevention
Use collars or fine mesh over the whole bed from planting day. Firm transplants in well, water them establishing quickly, rotate brassicas, and clear all old brassica roots after harvest so pupae do not overwinter in the bed.
Crops at risk
Related problems
Cabbage Root Fly risk by location
Each location has a month-by-month profile computed from the weather archive: how many hours a year actually meet this disease’s thresholds.