A review on clubroot of crucifers: symptoms, life-cycle of pathogen, factors affecting severity, and management strategies

Clubroot is a devastating disease of crucifers throughout the world. It is caused by a soil-borne obligate phytoparasite, Plasmodiophora brassicae Wor. Plant affected by this disease shows flagging of leaves, unthrifty growth, and even premature death. When uprooted, root shows characteristic symptom of hypertrophied club-shaped root system. Several biotic and abiotic factors affect the disease severity. Biotic factors include spore load in soil and virulence of pathogen, whereas abiotic factors generally include soil environmental factors such as soil temperature, soil pH, soil moisture, and soil type. Pathogen survives, for substantial period of time in absence of host, through its double-walled resting spores in soil or crop debris. Temperature affects spore germination, occurrence, and pathogen proliferation. Acidic soil reaction is crucial for pathogen to proliferate, metabolize, secret enzymes, and to complete life-cycle. All type of soil textures favor disease; however, severity differs with type of soil and soil organic matter content. Soil moisture provides platform to move bi-flagellated zoospores to infect root hairs of crops. Root hair infection is commensurate with inoculum density or spore load in soil. Immediate management strategies entail cultural practices, use of biocontrol agents, and application of chemical as last resort. Trichoderma spp., Pseudomonas fluorescens, Bacillus subtilis, and Gliocladium catenulatum are potential biocontrol agents. Flusalfamide, Fluazinam, and Cyazofamid are some common chemicals used to control clubroot. Soil carried by farm implements, human body, irrigation water, and flood can be potential source of pathogen. The risk of clubroot can be reduced by ensuring phyto-sanitory measures, destroying host crop debris, regular scouting, growing resistant cultivars, avoiding acidic soil reaction, eliminating weedy hosts, and reducing soil movement.