Clubroot is caused by the fungus Plasmodiophora. The fungus infects crucifers ( e.g: cauliflower, cabbage, broccoli and chinese cabbage) causing large knotted galls to form on the roots. The galls prevent the uptake of water and nutrients by the plant causing wilting in warm weather, which is the first noticeable symptom of clubroot infected plants. Severely infected plants are usually stunted and do not produce marketable crops.
Lancaster, Rachel; Donald, Caroline; and Porter, Ian
Journal of the Department of Agriculture, Western Australia, Series 4: Vol. 39
, Article 4.
Available at: http://researchlibrary.agric.wa.gov.au/journal_agriculture4/vol39/iss1/4