Clubroot

Plant disease · Plasmodiophora brassicae

Description

Clubroot is a soilborne disease of cabbage-family plants, including cabbage, broccoli, cauliflower, kale, Brussels sprouts, radishes, and turnips. It is caused by Plasmodiophora brassicae, a protist rather than a true fungus. The pathogen infects roots and triggers abnormal cell growth, forming swollen, misshapen clubs that interfere with water and nutrient uptake. Above ground, plants may look healthy at first, then grow slowly or wilt on warm days. Severe infections can prevent heads or edible roots from developing properly. The pathogen spreads mainly when contaminated soil moves on tools, shoes, transplants, or in water. Its resting spores can survive in soil for many years, so removing a sick plant does not remove the disease from that bed. Infection is favored by moist soil, particularly when the soil is acidic. Clubroot affects both young and established plants, though early infection generally causes the greatest losses.

Symptoms

• Galls or swellings
• Wilting
• Stunted growth
• Yellowing leaves

How to Identify

Lift a struggling cabbage-family plant carefully and wash soil from its roots. Clubroot produces enlarged, irregular swellings on the main and smaller roots; these may later turn brown and decay. Above ground, look for stunting, yellowing, and wilting that can temporarily improve overnight. Drought or damaged roots can cause similar wilting, but they do not produce the characteristic clubs. Root-knot nematodes also cause swellings, usually as smaller, more distinct knots, and can affect many plant families. Confirm uncertain cases through a local plant diagnostic laboratory, especially before treating a garden bed as permanently infested.

Prevention

Start with healthy transplants and clean potting mix. Do not move soil from an infested bed to clean beds; wash soil off tools, footwear, and equipment, and avoid working contaminated soil when it is wet. Improve drainage and avoid prolonged waterlogging. Test soil pH and, where appropriate, apply agricultural lime well before planting to raise it to around 7.2–7.5; follow the soil-test recommendation rather than adding lime indiscriminately. Grow resistant cabbage-family varieties when available. Rotate susceptible crops out of affected beds for at least five to seven years, recognizing that spores may persist longer. Remove cabbage-family weeds, which can host the pathogen, and keep infected plants and their soil out of home compost.

Treatment

There is no reliable cure for an infected plant. If only a few plants are affected, remove them promptly, including as much root material as practical, and put them in the trash rather than a home compost pile. Brush or wash contaminated soil from tools and shoes before entering other beds. Keep remaining plants evenly watered to reduce wilting, but do not leave the soil waterlogged; this may ease stress without eliminating the infection. For future crops, use resistant varieties, improve drainage, and adjust soil pH according to a soil test. Move susceptible crops to a clean bed if possible. A long rotation can reduce disease pressure, but it may not eradicate the long-lived spores. Soil solarization may reduce inoculum in some climates but is not consistently effective. Fungicide sprays are not a dependable home-garden treatment for clubroot.

Plants Affected

Arugula, Broccoflower, Broccoli, Broccolini, Brussels Sprouts, Cabbage, Cauliflower, Chinese Broccoli, Chinese Cabbage, Collard Greens, Daikon, Horseradish, Kale, Kohlrabi, Komatsuna, Mizuna, Mustard Greens, Napa Cabbage, Radish, Rapini, Red Cabbage, Romanesco, Rutabaga, Savoy Cabbage, Tatsoi, Turnip

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