Clubroot

Clubroot is one of the most stubborn diseases a vegetable grower can meet. It is caused by a soil-borne organism (Plasmodiophora brassicae) that infects the roots of plants in the cabbage family, making them swell into thick, distorted, club-shaped lumps. Those damaged roots can no longer take up water and nutrients properly, so the plant above ground grows slowly, wilts in warm weather even when the soil is moist, and often turns a sickly yellow or purplish colour. Badly affected plants barely crop at all.

Which crops it hits

Clubroot attacks all brassicas — the whole cabbage family. That means cabbage, kale, broccoli, calabrese, cauliflower and Brussels sprouts, plus the ones people forget are brassicas: turnips, swede, kohlrabi, radish, rocket and even some ornamental wallflowers. If a plant belongs to that family, clubroot can get it. Closely related weeds such as shepherd's purse can quietly keep the disease ticking over too, which is one reason it's so hard to shake.

Why it persists

The thing that makes clubroot so frustrating is its staying power. The disease produces resting spores that survive in the soil for a remarkably long time — commonly quoted as up to 20 years — waiting for a brassica root to come along. Once a plot is infected, you can't dig the spores out or kill them off in any practical way, so the disease is effectively there for the long haul. It spreads on muddy boots, tools, barrow wheels and water run-off, and on the roots of bought-in brassica plants, so it travels easily from one plot or allotment to the next.

Living with it: management

You can't cure clubroot, but you can keep growing decent brassicas by making conditions as unfavourable as possible:

• Raise the pH with lime. Clubroot hates alkaline soil, so add garden lime to bring the soil towards pH 7–7.5 before planting. This is the single most effective home measure.
• Improve the drainage. The disease thrives in wet ground, so dig in plenty of organic matter, and grow brassicas in raised beds on heavy, sticky soil.
• Rotate crops. Practising crop rotation won't shift established spores, but on clean ground it stops the disease building up in the first place.
• Grow resistant varieties. UK breeders now offer clubroot-resistant cabbages, cauliflowers, kale and swede (look for names such as 'Kilaton' and 'Clapton'), which crop reasonably even in infected soil.
• Start strong. Raise plants in pots so they go out with a good root system, and keep tools, boots and trays clean to avoid carrying spores about.

On a known infected plot, combining lime, sharp drainage and a resistant variety usually means you can still bring home a worthwhile harvest. For the full diagnosis and a season-by-season plan, see our guide to clubroot in brassicas.