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Most plant diseases are caused by fungi, bacteria, and viruses. Although the term disease is usually used only for the destruction of live plants, the action of dry rot and the rotting of harvested crops in storage or transport is similar to the rots of growing plants; both are caused by bacteria and fungi.
Rust, disease of thousands of economically important plants, as well as weeds, caused by more than 4,000 species of fungi. 1  During their life cycle rust fungi parasitize either one species of plant (autoecious, or monoecious, rust) or two distinct species (heteroecious rust). One heteroecious rust with five spore forms during its life cycle is black stem rust (Puccinia graminis) of wheat and other cereals and grasses.