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.
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.