Rust

Rust, in botany, fungi of the order Uredinales and the plant diseases they cause. Rusts infect their hosts by forming orange or red spots, their spore-bearing organs, on their host's leaves. Some rusts are heteroecious: they alternate between host plants of 2 different species (e.g., the cedar rust, which infects apple and cedar trees). One crucial rust fungus (Puccinia graminis) attacks grain crops, causing black-stem rust of wheat. Rusts also attack ornamentals, fruits, and vegetables.