Definition of 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
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