Black rust of wheat is caused by

Black rust of disease is caused by a fungus, member of basidiomycetes Puccinia graminis.

In this, brown coloured pustules spread on both side of stem leaves and spikes

A, C, D are wrong because.

• Ustilago nada cause smuts.
• Alternaria solani cause early blight of potato.

Xanthomonas oryzae cause bacterial blight

Option B is the correct answer, that is Black rust of wheat is caused by

Puccinia graminis