Rust and Smut, plant diseases caused by a number of parasitic types of fungi and funguslike protists. These fungi and protists are also called rusts and smuts. Rusts and smuts are the most destructive diseases of wheat, but they attack many other kinds of plants also.
Rusts are named for the usually rusty appearance of their spores, the dustlike bodies by which they reproduce. Most rusts produce more than one kind of spore during their life cycles; some produce as many as five kinds. A typical rust, in its life cycle, infests first a plant of one species and then a plant of another species.
Stem rust is responsible for the most severe losses of wheat crops. Stem rust forms elongated reddish-brown pustules. It grows on and breaks through the surface of the stems and leaves of wheat and other plants. Crops attacked by stem rust are usually not worth harvesting. The plants become dried, ripen prematurely, and produce lightweight, shriveled grain because the rust fungus absorbs large amounts of water and nutrients from the plant. White rust forms white pustules. It grows on the leaves and stems of a number of crops, including spinach, cabbage, radishes, and sweet potatoes.
Crop losses caused by rusts can be reduced by planting rust-resistant varieties of crops, by dusting or spraying crops with fungicides, and by soaking seeds in fungicides.
Smuts are named for the dark, dirty appearance of their spores. Smuts form masses of spores on flowerheads, stems, leaves, or seeds. The spores of loose smuts are readily scattered by the wind. The spores of covered smuts are bound together in a solid mass. When the mass is shaken, as by threshing, the spores are released and easily scattered. Smuts attack few plants except cereals. Stinking smut, or bunt, smells like rotten eggs; it attacks wheat.
Crop losses caused by smuts can be reduced with methods similar to those for reducing crop losses caused by rusts.
Most rusts belong to the order Uredinales of the division Basidiomycota. White rusts belong to the order Peronosporales of the phylum Oomycota. Smuts belong to the order Ustilaginales of the division Basidiomycota.