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 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 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