Clubroot on cauliflower — symptoms and treatment

Severity: high

What is Clubroot

Clubroot in cauliflower is caused by Plasmodiophora brassicae. It is one of the most serious brassica diseases. The organism lives in the soil and infects the roots, where it triggers the swollen, club-shaped galls that give the disease its name. Those deformed roots cannot move water and nutrients properly, so the plant above ground starves even when the soil is moist — which is why clubroot so often ends in a small curd, or no curd at all.

Symptoms

The first sign is usually a crop that wilts during the day and recovers overnight, as if it were short of water. As the root system is progressively replaced by galls, the plants stay stunted, and cauliflower that never builds a proper root system never builds a proper head. Pulling up a suspect plant is the diagnostic step that settles it: healthy fibrous roots are replaced by swollen, club-shaped or spindle-shaped growths. Later these galls rot and break down, sometimes with a foul smell.

• Early: wilting during the day, recovery at night, uneven growth across the bed
• Early: plants lag behind their neighbours for no obvious reason
• Advanced: swollen, club-shaped roots visible when the plant is lifted
• Advanced: stunted growth; small or no curd formation

Above-ground wilting and stunting alone can be mistaken for drought, root damage from cultivation, or cabbage root fly. Only lifting the plant and looking at the roots distinguishes clubroot — cabbage root fly leaves tunnelled, chewed roots rather than smooth swollen clubs.

Causes and conditions

Plasmodiophora brassicae survives in soil as resting spores that persist for many years without a brassica crop present, which is what makes clubroot so difficult to shake off once a plot has it. The spores germinate and infect root hairs when the soil is wet, so waterlogged and poorly drained ground favours the disease strongly. It travels in anything that moves soil: boots, tools, tyres, machinery, and surface water running across a field. The most common way it arrives on a clean plot, though, is on infected transplants raised in contaminated soil. All brassicas can host it, including weeds in the same family, so volunteer brassicas and related weeds keep the population alive between crops.

Treatment

There is no spray that cures an infected plant — clubroot management is cultural and it is done before planting.

Liming and resistant varieties — cultural

Timing: pre-planting. Raise soil pH to 7.2+ with lime. Use resistant varieties. Long rotation with non-brassicas (7+ years).

Prevention

• Use certified, disease-free transplants; never bring in plants raised in unknown soil.
• Keep a long rotation with non-brassica crops, and count brassica weeds as part of the rotation.
• Improve drainage on heavy or waterlogged ground so root hairs are not sitting in wet soil.
• Clean soil off boots, tools and machinery before moving from an infected plot to a clean one.
• Lift and remove affected plants with their root galls rather than tilling them back in.

Frequently asked questions

Is clubroot contagious to my other plants? It only affects brassicas — cauliflower, cabbage, broccoli, kale, turnips, radishes and their wild relatives. Tomatoes, beans or onions in the same bed are not at risk. But the soil itself becomes the reservoir, so the next brassica crop planted there is.

Can I still eat the cauliflower? Clubroot is a root disease of the plant and is not a human health issue. In practice the problem is that badly affected plants form a small curd or none at all.

Once I have it, how long does it stay in the soil? Resting spores are long-lived, which is why the recommendation is a long rotation with non-brassicas of 7+ years alongside liming and resistant varieties. Treat an affected plot as contaminated for the long term rather than expecting a quick fix.