black root

1 : any of several plants or their dark-colored roots: such as

a : a perennial plant (Pterocaulon undulatus) of the southern U.S. that has large black rootstocks

b : culver's root

c : colicroot sense 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: such as

a : cotton wilt sense 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