Clubroot on broccoli — symptoms and treatment

Severity: high

What is Clubroot

Clubroot in broccoli is caused by Plasmodiophora brassicae, a soil-borne organism that attacks the roots. It persists in acidic soils for many years, which is what makes it such a serious problem: this is not a disease you fix in one season, it is a change to the ground itself. The pathogen deforms the root system so the plant cannot take up water and nutrients properly, and once the ground is infested every brassica planted there is at risk. High severity is justified by the persistence, not just by the damage to a single crop.

Symptoms

Above ground, the first thing you notice is wilting — plants that flag despite the soil being moist, because the damaged roots cannot supply the top of the plant. Leaves turn yellow and growth stalls. Broccoli in particular pays for it at harvest: the heads come small and the florets are poor quality, even on plants that look like they are limping along well enough. The diagnosis is confirmed underground. Pull a suspect plant and the roots are swollen and distorted into thickened, club-like growths instead of a normal fibrous root system.

• Above ground: wilting; yellowing leaves; stunted, slow growth.
• At harvest: small heads; poor quality florets.
• On the roots: swollen, distorted, club-shaped growths.

Wilting and yellowing on their own look like drought, waterlogging or a nutrient shortage. The swollen, distorted roots are the definitive sign — always check the roots before concluding anything else.

Causes and conditions

The pathogen lives in the soil as long-lived resting spores, which is why an infested bed stays infested for many years even with no brassicas grown in it. Spores germinate and infect brassica roots, the roots respond by swelling into galls, and when those galls break down at the end of the season they release a fresh load of spores into the ground. Acidic soils favour the disease — the pathogen persists and infects most readily where the pH is low. Wet, poorly drained soil helps it too, since the infective stage moves through soil water. The spores travel on anything that carries soil: boots, tools, tyres, and above all infected transplants, which is a common way clubroot is introduced into a clean garden.

Treatment

Liming and rotation — cultural

Timing: pre-planting. Raise soil pH to 7.2+ — the pathogen is favoured by acidic conditions, so liming the ground before planting is the central measure. Rotate with non-brassicas for 7+ years to starve the resting spores of a host. Use resistant varieties if available.

There is no spray that cures clubroot once a plant is infected; the work is done in the soil before planting.

Prevention

• Test the soil and lime ahead of planting to raise the pH to 7.2+ where brassicas will grow.
• Rotate with non-brassicas for 7+ years on any ground where clubroot has appeared.
• Raise or buy transplants from a clean source — infected transplants are a common way the pathogen arrives in a new garden.
• Improve drainage and avoid working wet, infested soil; clean soil off boots and tools when moving between beds.
• Choose resistant varieties where they are available for your growing conditions.

Frequently asked questions

Can I treat the plants once I see the clubbed roots? No. By the time the roots are galled, the damage is done and that crop will not recover. Pull and destroy affected plants and put the effort into liming and rotation before the next brassica crop goes in.

Can I grow anything else in that bed? Yes. Clubroot only affects brassicas — cabbage, cauliflower, kale, turnips, radish and brassica weeds. Non-brassica crops grow normally in infested ground, which is exactly what makes a long rotation practical.

Will liming get rid of it completely? Liming to raise the pH suppresses the disease and makes brassicas growable again, but it does not sterilise the soil. The resting spores persist for many years, so keep up the rotation and use resistant varieties alongside liming rather than relying on lime alone.