Puccinia graminis causes
Puccinia graminis f. sp. tritici is a macrocyclic heteroecious basidiomycete responsible for black stem rust of wheat, historically causing major famines. Dikaryotic urediniospores on wheat stems produce reddish-brown pustules enabling polycyclic secondary spread through wind dispersal, teliospores survive winter, germinate to form promycelium producing haploid basidiospores infecting alternate host Berberis vulgaris. Barberry supports pycnial spermatization and aecial stages providing sexual recombination generating new races. Haustorial penetration through stomata collapses photosynthetic tissue, shrivels grains, reduces yield dramatically distinct from corn smut or fungal blights.
Ref: Agrios Plant Pathology 5th ed. Chapter 11 Rusts; NCERT Class 12 Biology Chapter Plant Diseases