noun
1. : any of several plants or their dark-colored roots: as
a. : a perennial plant ( Pterocaulon undulatus ) of the southern United States that has large black rootstocks
b. : culver's root
c. : colicroot 1
d. : a common comfrey ( Symphytum officinale ) with the upper part of the stem and inflorescence densely hispid
2. : any of several plant diseases characterized by dark discoloration of roots: as
a. : cotton wilt 1
b. : a disease of radishes, beets, and sugar beets caused by fungi (genus Aphanomyces ) and producing brown or black lesions on the roots, deformation of the roots, and a damping off of seedlings