Rusts are microscopic fungi and are strictly parasitic, producing disease in grasses and cereal crops. In the USA alone, about 125 different species of rusts attack grasses and nearly 400 species of grasses are among the hosts of rusts. Some of the rusts attack only one of a few grasses but others can attack a great many. Similarly, some of the rusts of grasses are also destructive to cereals and most of the so-called cereal rusts have numerous grass hosts.
Because the rust fungi are parasites, their development on a host plant is at the expense of that plant and the nutrients that they take would otherwise go into seed, forage, or both. A light infestation of rust is not likely to cause noticeable effects on yield of seed or forage, but a heavy infestation definitely will. In grasses grown for seed, a heavy infestation will result in low test weight of the seed because of the direct effect of parasitism in sapping nutrients from the host and because of water loss through the numerous open rust pustules on the leaves and stems. Rust likewise affects the production of grasses for forage, chiefly in lower yields as a result of reduced vigour.
Secondary or indirect adverse effects may also occur. Heavy attacks of rusts on grasses will make them more likely to succumb to other factors that are always more severe on the already weakened plant - drought, winter injury, root rot, snow mould, and perhaps other diseases.
Although rusts extensively attack grasses in general, it is on cereal crops that they inflict the greatest economic damage. Stem rust of wheat causes the most spectacular and perhaps the greatest losses, which range up to 85 and 90%. Leaf rust of wheat and crown rust of oats occur more frequently and affect large acreages, and so may cause greater average losses year in and year out. Furthermore, leaf rust of wheat and crown rust of oats occur wherever wheat and oast, are grown. Crown rust is the most destructive disease of oats, cutting yields by 20 to 50%. Stem and leaf rusts are the most destructive diseases of wheat in North America, whereas in Europe, stripe rust is the most common and most destructive. Besides wheat, it also attacks barley and rye, and it occurs in Africa, South America, Japan, China and India.
Wheat, oats, barley, and rye may be attacked by eight distinct species or subspecies of rust fungi. Wheat is subject to stem rust Paccinia graminis tritici, leaf rust P rubigovera, and stripe rust P glumarum. Oats are attacked by a stem rust P graminis avenae and by crown rust P coronata avenae. Barley may fall prey to the same stem rust that attacks wheat P graminis tritici and a leaf rust P hordei. A stem rust P graminis secalis and a leaf rust P rubigovera attack rye. Each of the eight rusts is made up of several or many different races, which may attack certain varieties of a particular cereal crop but not others.