Clubroot on cabbage — symptoms and treatment

Severity: high

What is Clubroot

Clubroot is caused by Plasmodiophora brassicae, a soil-borne organism that infects the roots of cabbage and its brassica relatives. Infection makes the roots swell into gnarled galls, and those galls prevent nutrient uptake — the plant is left with a root system that looks substantial but functions poorly. Because the pathogen lives in the soil and survives there for years, clubroot is a problem attached to the ground rather than to a single crop, which is why it rates high severity.

Symptoms

The tell-tale above-ground symptom is wilting on hot days even when the soil has adequate moisture. This is the giveaway that separates clubroot from simple drought: water is there, but the galled roots cannot move enough of it to keep up with demand when the sun is strong. Plants often perk up again overnight and wilt once more the next hot afternoon. Alongside this, growth is stunted and the leaves yellow, frequently starting with the older, outer leaves. Heads stay small or fail to form properly.

• Early: wilting on hot days despite adequate soil moisture; recovery overnight.
• Progressing: stunted growth; yellowing leaves; small or poorly formed heads.
• Confirmed underground: swollen, club-shaped roots in place of normal fibrous roots.

Because the top-growth symptoms mimic drought, waterlogging and nutrient deficiency, never diagnose clubroot from the leaves alone. Lift a struggling plant and look at the roots — the clubbed, distorted swellings are unmistakable.

Causes and conditions

The pathogen survives in the soil as tough resting spores that remain viable for many years, which is why once a bed has clubroot it keeps it. Those spores germinate near brassica roots and infect them; the roots swell into galls; and when the galls decay they release another generation of spores, so each infected crop makes the problem worse. Acidic soil favours the pathogen, and wet, poorly drained ground helps it because the infective stage swims through soil water. It spreads on anything carrying infested soil — boots, tools, wheelbarrows, run-off — and most often arrives in a clean garden on infected transplants.

Treatment

Liming and resistant varieties — cultural

Timing: pre-planting. Raise soil pH to 7.2+ with lime — the pathogen thrives in acidic conditions, so shifting the pH is the core measure and it has to be done before planting, not after symptoms appear. Use resistant varieties. Rotate with non-brassica crops for 7+ years to deny the resting spores a host.

No spray will cure a plant whose roots are already clubbed. Everything effective happens in the soil before the crop goes in.

Prevention

• Test the soil and lime in advance to bring the pH to 7.2+ wherever cabbage will be grown.
• Rotate with non-brassica crops for 7+ years on any bed with a clubroot history.
• Use resistant varieties, and raise your own transplants or buy from a clean source — infected transplants are the usual way clubroot enters a garden.
• Improve drainage; avoid working infested beds when wet, and clean soil from boots and tools before moving to clean ground.
• Pull and destroy infected plants with their roots intact — leaving galls in the soil to rot feeds the next generation of spores.

Frequently asked questions

My cabbage wilts every afternoon but the soil is wet — is it clubroot? It is a strong indicator. Wilting on hot days with adequate moisture is the classic clubroot picture, because galled roots cannot supply water fast enough. Confirm by lifting a plant and checking for swollen, club-shaped roots.

Can I still grow other vegetables in that bed? Yes. Clubroot only attacks brassicas — cabbage, broccoli, cauliflower, kale, turnips, radish and brassica weeds. Everything else grows normally, which is what makes the long rotation workable.

Does liming cure the soil? It suppresses the disease rather than eliminating it. Raising the pH to 7.2+ makes conditions hostile to the pathogen and lets you grow a crop, but the resting spores persist for years. Combine liming with resistant varieties and the rotation instead of relying on it alone.