Clubroot
Plasmodiophora brassicae
Overview
Clubroot is caused by Plasmodiophora brassicae, a soilborne protist that infects brassicas such as cabbage, broccoli, cauliflower, kale, and brassica cover crops. Resting spores can survive in soil for a decade or more. Cool, wet, acidic soils favor infection of root hairs. Once clubs form, roots cannot supply water efficiently and plants wilt and stunt. Because spores persist so long, prevention and resistant cultivars matter more than trying to eradicate the organism after it is established.
Symptoms
Roots develop swollen, spindle-shaped or knobby clubs that later decay into a foul mess. Infected transplants wilt on warm days and may partially recover at night. Plants remain stunted, yellow, and fail to form quality heads. Young plants can be killed outright. When dug, the distorted root system is unmistakable. Above-ground symptoms mimic drought even when soil moisture is adequate. Patches in beds often expand slowly each brassica season.
Environmental Conditions
Humid
Prevention
Never introduce soil or shared transplants from unknown brassica beds. Clean tools and boots after working in infested areas. Start seed in clean mix rather than garden soil. Avoid moving irrigation water that carries soil between plots. Liming and resistant varieties are the core prevention package in acidic, wet climates where clubroot is established. Keep wild mustard relatives down near production beds.
Management
Raise soil pH toward neutral to slightly alkaline (often near 7.2) where that fits your system, using lime carefully and retesting. Plant clubroot-resistant brassica cultivars and rotate resistance sources. Improve drainage and avoid compact, waterlogged beds. Remove and destroy clubbed roots; do not compost them in ways that return spores to beds. Extend rotations away from all brassicas, including mustards and brassica cover crops, for as many years as practical on infested ground.