Some of the 125 Rusts of Grasses

George W. Fischer.

About 125 different species of rusts attack grasses in the United States. Nearly 400 species of grasses are among the hosts of the rusts.

Some of the rusts attack only one or a few grasses. Others can attack a great many. Stem rust, Puccinia graminis, for example, has been recorded on nearly 200 species of grasses in the United States. Among the rusts, which have such large numbers of hosts, we find innumerable strains or races, which look alike, even under the microscope, but differ in their comparative ability to attack species or varieties of grasses.

Some of the rusts of grasses are often destructive on the cereals. Most of the so-called cereal rusts have numerous grass hosts; probably the cereal rusts originally were grass rusts that found susceptible hosts among the cereals during the centuries they have been cultivated by man. However, since the cereals wheat, oats, barley are really only grasses whose seeds are large enough and nutritious enough to warrant intensive cultivation as food, it is only to be expected that there would be a great deal of similarity between the rusts that attack grasses and those that attack cereals.

THE RUSTS are microscopic fungi and are strictly parasites. They are among that parasitic classification known as obligate parasites; that is, they can grow only on a living plant. Indeed, many of the rust fungi require not only a certain species of grass or cereal but frequently a definite strain or variety of that species a phenomenon known as host specialization, host specificity, or physiologic specialization. It has an important part in breeding crop plants for resistance to rust.

Because the rust fungi are parasites, their development on a host plant is at the expense of that plant and the nutriments 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 vigor. That, however, is partly offset by the somewhat higher protein content of rust-infested grass over rust-free grass.

Secondary or indirect adverse effects should not be overlooked. 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 mold, and perhaps other diseases.

Reproduction and dissemination in the rust fungi, as in fungi generally, is by microscopic cells or groups of cells known as spores. Many of the rusts produce several kinds of spores during their life cycle. Very common are the familiar red uredospores, whose mass appearance accounts for the name rust. Those spores are borne throughout the summer and spread the disease rapidly during the growing season. As the plants mature, however, the red pustules gradually change to a black type. The black spores (teliospores) usually cannot germinate until the following spring; thus they insure survival of the rust fungus through fall, winter, and early spring.

IN MANY of the rusts, especially the grass and cereal rusts, the black spores cannot infect any grass or cereal host but only some entirely different kind of plant, as the examples given later will show. Such a plant is known as the alternate host and usually is infected in the spring or early summer. The spores produced on the alternate host cannot reinfect that host; instead, they give rise to the initial red spore stage again on grasses or cereals, and thus the life cycle is completed.

Few of the 125-odd species of rust fungi that attack grasses in the United States are of economic importance: Many of the rusts are so limited geographically that they have not presented any general problem. Many are specialized to a few grasses, frequently of limited distribution and importance in the over-all range and forage picture. Another circumstance: In many wild grasses, especially of the bunch type, individual plants of a species often are separated by many other types of vegetation and by resistant grasses of other species. Susceptible individuals of one species have to be close by before rusts and leaf diseases in general can develop into an epidemic. That has been demonstrated countless times and often with disastrous results in cultivated fields of cereals and grasses.

Nevertheless, several rusts often reach economic proportions in important forage grasses and merit specific mention here. They are stem rust, leaf rust, stripe rust, crown rust, and bluegrass rust.

STEM RUST OF GRASSES (Puccinia graminis) is better known as a destructive disease of small-grain cereals than as an important disease of numerous grasses. It is evident primarily as small red pustules, chiefly on the stems, but sometimes also on the leaves, particularly on young grass plants. As the summer draws to a close and the plants mature, the red pustules gradually are replaced by black ones.

The six varieties of the stem rust fungus in the United States are differentiated mostly by the cereals and grasses they can attack. Thus the wheat variety of stem rust (Puccinia graminis tritici) attacks wheat, barley, rye, and many grasses. The oats variety (P. graminis avenge) attacks oats and various grasses. The rye variety (P. graminis secalis) parasitizes not only rye but also certain grasses. The redtop variety (P. graminis agrostidis) occurs on redtop and related grasses. The bluegrass variety (P. graminis poae) is on Kentucky bluegrass and many other bluegrasses. The timothy variety (P. graminis phlei-pratensis) attacks timothy and a few other grasses.

The wheat variety of stem rust has been found on many of the wheat-grasses (Agropyron species), orchard-grass, wild-rye grasses (Elymus species), barley grasses (Hordeum species), and a few less important grasses. The stem rust of oats occurs on tall oatgrass, sweet vernalgrass, some brome grasses, orchardgrass, a few fescues, canary-grass, timothy, a few bluegrasses, and some others of less importance. Stem rust of rye attacks many of the wheat-grasses, a few bromes, many of the wild-rye grasses, and barley grasses. The redtop variety is practically restricted to redtop and related grasses (Agrostis species). Stem rust of blue-grasses likewise is restricted to Kentucky bluegrass and a few other of the bluegrasses. The timothy variety is virulent on tall oatgrass, orchardgrass, meadow fescue, and tall fescue. So it is apparent that three of the varieties of stem rust attack both cereals and grasses and that the other three are restricted to grasses only.

In the Southern States and other places of mild winters, stem rust usually overwinters in the red stage on grasses and on volunteer or fall-sown grain. In regions of colder winters, such overwintering is exceptional, and the rust fungus depends on the black spore stage for survival during fall and winter and for reestablishment in the following early summer. Reestablishment is impossible without intervention of a spring stage of stem rust on an entirely unrelated plant, the common barberry (species of Berberis), the alternate host. The resulting infections on barberry bushes produce spores that set up the initial red spore stage on cereals and grasses in early summer. It is this stage that builds up in epidemic form by repeated generations during summers of frequent rains or prolonged dew.

LEAF RUST (Puccinia rubigo-vera) occurs mainly on the leaves of cereals and grasses, although stem infections are encountered sometimes. Host specialization occurs, similar to that described for stem rust, so that we have six varieties, as follows:

One variety of leaf rust, Puccinia rubigo-vera agroppri, common in the Rocky Mountain region, infects many of the wheatgrasses, various bromes, and several of the wild-rye grasses. It has many alternate hosts, all in the buttercup family (clematis, buttercup, columbine, larkspur, and others).

Another rust variety, P. rubigo-vera apocrypta, also occurs on a few wheat-grasses and wild-rye grasses but has alternate hosts in the waterleaf family (e. g. Hydrophyllum) and the borage family (e. g. Mertensia).