Which of the following is NOT a method of controlling clubroot disease?

Systemic fungicides are not an effective method for controlling clubroot disease, which is caused by the pathogen Plasmodiophora brassicae. This disease primarily affects brassica plants, and its control typically relies on cultural practices rather than chemical treatments.

Soil solarization, implementing fallow periods, and monitoring pH levels are strategies that can help manage or reduce the incidence of clubroot disease. Soil solarization involves covering the soil with transparent plastic to trap solar energy, heating the soil and reducing pathogens, including clubroot. Fallow periods allow for the depletion of the clubroot pathogen in the soil by preventing host plants from growing, thereby breaking the disease cycle. Monitoring and adjusting soil pH levels can also be crucial, as clubroot thrives in acidic conditions; raising the pH can help suppress the pathogen's activity.

Thus, while cultural practices can effectively manage clubroot, systemic fungicides do not target the pathogen involved and are not recommended for controlling this specific disease.