Clubroot

Clubroot is caused by Plasmodiophora brassicae, a soil-borne pathogen that infects the full range of brassica crops including cabbage, broccoli, kale, radish, and turnips. Though categorized as a fungal disease in most practical guides, it is technically a protist. That distinction matters less than its most consequential characteristic: resting spores can persist in soil for 10 to 20 years. Once a bed or plot is contaminated, the pathogen is essentially permanent.

Infection begins when resting spores germinate in moist soil and release zoospores that penetrate root hairs. The pathogen redirects cell division in root tissue, producing the distorted, club-shaped swellings that give the disease its name. These malformed roots lose their ability to move water and nutrients efficiently, causing plants to wilt during warm parts of the day, yellow at the lower leaves, and stunt overall. Infected plants pull from the soil with little resistance because root structure is compromised.

Conditions that favor infection include acidic soil (pH below 7.0), poor drainage, and soil temperatures in the range of 64°F to 77°F. Heavy clay soils that stay waterlogged after rain are particularly hospitable. The pathogen spreads readily through contaminated soil on tools, boots, and purchased transplants, making clean sourcing a real preventive factor.

Raising soil pH to 7.2 or above with lime is the most reliably documented management strategy. Alkaline conditions suppress spore germination and slow infection progression substantially. Resistant varieties are commercially available for several brassica crops and are worth prioritizing on any ground with a known history of the disease.