Black stem rust of wheat is caused by Xanthomonas oryzae, Ustilago nuda, Alternaria solani, Puccinia graminis.

The stem, black and cereal rusts are caused by the fungus Puccinia graminis. P. graminis is a member of the phylum Basidiomycota within the kingdom Fungi. The characteristic rust colour on stems and leaves is typical of a general stem rust as well as any variation of this type of fungus. The stem rust fungus attacks the parts of the plant which are above ground. Spores that land on green wheat plants form a pustule that invades the outer layers of the stalk. Infected plants produce fewer tillers and set fewer seed, and in cases of severe infection the plant may die. Thus, the correct answer is option D.