a disease of plants of the cabbage family, caused by a slime mold (Plasmodiophora brassicae) and characterized by swellings of the roots
See club root in American Heritage Dictionary 4
noun
A disease of cabbage and related plants, caused by a fungus (Plasmodiophora brassicae) and characterized by knobby or club-shaped swellings on the roots and wilting, yellowing, and stunted growth of aboveground parts.