Clubroot, carrot fly and onion white rot
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Rotation helps against carrot fly, but it can't clear clubroot or onion white rot once they're in the soil: their spores last 15 to 20 years. For those two, keep them out by not moving infected soil.
These three live in or around the soil, and they're the reason allotment-holders talk so much about rotation. Rotation helps with some of them. With two of them, the best defence is never letting them in.
Clubroot (cabbage family)
What it is: a soil-dwelling organism, Plasmodiophora brassicae, that attacks the roots of brassicas. It also affects wallflowers, stocks and weeds such as shepherd's purse.
Signs: plants are stunted, leaves may turn purplish and wilt on hot days, then perk up when it cools. Dig one up and the roots are swollen and distorted, with the fine roots gone.
Why it's a problem: its resting spores can contaminate soil for up to 20 years. Most new infections happen from mid-summer to late autumn, when the soil is warm and moist.
What helps:
- Lime acid soil to raise the pH. It reduces clubroot but doesn't get rid of it.
- Improve drainage, for example with raised beds.
- Grow plants on in pots at least 9cm across before planting out, so they've a big healthy root system first.
- Choose resistant varieties. Some cultivars of sprouts, calabrese, cabbage, cauliflower and swede show some resistance.
- Don't move soil about. Clean tools, wheelbarrows and boots, and only buy plants from a source guaranteed free of clubroot.
There's no chemical control for gardeners. Moving brassicas round the plot won't clear it from infected soil, because the spores outlast any normal rotation.
Carrot fly (carrots and relatives)
What it is: a small black fly whose larvae feed on the roots of carrots, parsnips, parsley, celeriac and celery.
Signs: rusty brown scars ringing the roots. Cut one open and you'll find tunnels, often with slender creamy-yellow maggots.
Timing: eggs are laid in late May to June, and again in August to September.
What helps:
- Cover the crop completely with insect-proof mesh. RHS research found a complete covering is more effective than a barrier. See netting and mesh.
- Sow thinly, so you don't need to thin. Thinning can attract egg-laying females.
- Time it. Later sowings miss the first generation, and carrots lifted before late August miss the second.
- Rotate. Adults can emerge from pupae that overwintered in the soil, so don't grow carrots in the same bed two years running.
- Try less susceptible varieties. The RHS names 'Fly Away', 'Maestro', 'Resistafly' and 'Sytan', though none are fully resistant.
Of the three, this is the one where rotation clearly helps.
Onion white rot (onion family)
What it is: a soil-borne fungus that attacks onions, garlic, leeks and other alliums.
Signs: leaves yellow and wilt, especially in dry weather. The roots rot and the fungus gets into the bulb.
Why it's a problem: its resting bodies (sclerotia) can survive in soil for at least 15 years, so rotation isn't practical once it's in. There are no chemical treatments for UK gardeners.
What helps: keeping it off clean ground. It travels in soil on tools and muddy boots, so don't share tools between an infected bed and clean ones without cleaning them. Lift and bin infected plants rather than composting them, as new sclerotia form on rotting bulbs.
What rotation can and can't do
| Problem | Does rotation help? |
|---|---|
| Carrot fly | Yes, alongside mesh and timing |
| Clubroot | It slows build-up, but can't clear infected soil |
| Onion white rot | Not practical once present |
Rotation's main job is to stop crop-specific pests and diseases building up. The planner below plans a 4-year rotation for your beds and shows where each group goes next.
FAQs
How long does clubroot stay in the soil?
Up to 20 years.
Will crop rotation get rid of onion white rot?
No. Its resting bodies last at least 15 years, so rotation isn't practical. Keep it out of clean ground instead.
What's the best way to stop carrot fly?
Cover the crop completely with insect-proof mesh, sow thinly and time your sowings.
Your next step
Map which beds have had brassicas, carrots and onions in recent years, then plan the next four years in the planner on this page.
Allotment plot planner
Worked example
For a half plot (5 poles, about 126 m²), a quarter already clear, 4 hours a week, a shed and compost bays:
- 10 beds of 1.2 m with 45 cm paths, beds running across a plot about 19.5 m by 6.5 m
- Rotation: 8 beds in four groups, moving one step each year
- Broad beans: room for about 68 plants; Onions (sets): room for about 256 plants; Cabbage: room for about 68 plants; Carrots: room for about 1080 plants; Potatoes: room for about 86 plants; Courgettes: room for about 14 plants
- Clearing: about 95 m² to clear, roughly 12 weeks at 4 hours a week (our rule of thumb of 2 m² an hour)
- A 5 cm compost mulch over all the beds: about 3.9 m³, or 78 bags of 50 litres
Based on: National Allotment Society (NSALG), RHS. Crop spacing from RHS grow-your-own guides.
Sources
- National Allotment Society (NSALG): A traditional full allotment plot is 10 poles (rods or perches), about 250 square metres checked 24 Oct 2026
- RHS: RHS crop rotation: group crops (brassicas, legumes, onions, potato family and roots) and move each section a step forward every year to limit soil pests and diseases checked 24 Oct 2026
- RHS: RHS carrot fly: larvae tunnel into carrots, parsnips, parsley, celeriac and celery; eggs laid late May-June and August-September; sow thinly to avoid thinning; late sowing or lifting before late August avoids a generation; a complete cover of insect-proof mesh works better than a barrier; some cultivars are less susceptible awaiting check
- RHS: RHS clubroot: Plasmodiophora brassicae; swollen, distorted roots and stunted, wilting plants; resting spores contaminate soil for up to 20 years; liming and better drainage reduce it but don't eliminate it; raise transplants in pots at least 9cm; some resistant cultivars; avoid moving soil on tools and boots; no chemical control for gardeners awaiting check
- RHS: RHS onion white rot: soil-borne fungus of onions, garlic, leeks and other alliums; yellowing and wilting foliage; sclerotia survive in soil for at least 15 years; spread in soil on tools and footwear; rotation impractical; no chemical treatment available to UK gardeners awaiting check
- RHS: RHS: clubroot can contaminate soil for up to 20 years; liming and good drainage help but do not eliminate it checked 24 Oct 2026
- RHS: RHS fleece and crop covers: insect-proof mesh is a type of crop cover that keeps insects out, often used without hoops; insect-pollinated crops such as strawberries and courgettes are unsuited to covers while flowering awaiting check