Black rust

Black rust is a disease that only occurs on wheat. It is caused by a fungus, Puccinia graminis. Puccinia is a genus of the order of the Rusts (Uredinales). The telia, in which the teliospores* are formed, are black. There are also other rust diseases, such as the yellow and brown rust, but the black variant is the most aggressive. The first sign that wheat has been contaminated with it, is a type of red, rusty blisters on the stem. Then the wheat grains shrivel, the stem bends, and eventually the plant dies and turns black.

If you, as a medieval farmer, found this disease in your fields, you were sure that your wheat crop would fail. That often meant starvation; just like now, wheat was the staple food for many people at the time. 

*Teliospore, also called teleutospore, in fungi (kingdom Fungi), a thick-walled, winter or resting spore of rust fungi (phylum Basidiomycota) borne in a fruiting structure (telium) from which a club-shaped structure (basidium) is produced.

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