Not much is known regarding resistance of corn hybrids to head smut. In sorghum, where the disease is more important, resistant varieties are known and their use provides a means of control. Seed treatment and rotation are partly effective in reducing the prevalence of head smut in corn.

CORN IS SUSCEPTIBLE to three rust diseases common corn rust, southern corn rust, and tropical corn rust. In the United States only common corn rust and southern corn rust are known. Both diseases are generally of minor importance, but occasionally they may become severe and cause considerable leaf killing. Infection that becomes severe by Bilking time probably causes some actual loss in grain yield, but no accurate data are available on this point. In Mexico and Central America, corn rusts are frequently more severe and may prematurely kill plants. Tropical corn rust has been found only in the American Tropics. The three corn rusts differ somewhat in symptoms and in the morphology of the causal organisms.

COMMON CORN RUST is recognized by the appearance of circular to elongate, cinnamon-brown, powdery pustules scattered over both surfaces of the leaf. Their powdery nature is due to the masses of uredospores that form beneath and break through the epidermis of the leaf. As corn matures in the fall, black pustules are formed on both surfaces of the leaf because of formation of black teliospores in place of brown uredospores. Pustules will form on any above-ground part of the plant, but they are most abundant on the leaves.

Puccinia sorghi is the fungus that causes the common corn rust. Uredospores, or summer spores, are cinnamon brown, globoid to ellipsoid, and finely to moderately spined. The spores are spread about by wind. Under favorable conditions of moisture and temperature they germinate and penetrate the corn leaves. During the growing season, uredospores continue to produce new infections. Toward maturity of the host, teliospores are formed in the pustules. These spores are brownish black, oblong to ellipsoid, and rounded at both ends.

The teliospores are two-celled, and each is attached to a colorless pedicel once or twice the length of the spore. Teliospores overwinter and germinate in the spring to form small, colorless, thin-walled secondary spores, called basidiospores.

The basidiospores cannot attack corn, but are able to parasitize a number of species of Oxalis, or woodsorrel; the alternate host of the fungus. Following invasion of the woodsorrel leaves by the basidiospores, small, inconspicuous, pimplelike eruptions (spermagonia) are formed on both surfaces of the leaves. These spermagonia contain minute, colorless spores, called spermatia. The spermatia from one spermagonium must fuse with the hyphae that protrude from a spermagonium of opposite mating type before the fungus can continue growth and complete its life cycle. When such fusion takes place, the nuclei of opposite type associate in pairs in the mycelium, which then continues to grow and finally forms the cluster cup (aecial) stage of the fungus on the lower surface of the woodsorrel leaves. The cluster cups, called aecia, contain numerous aeciospores. These spores are globoid, finely spined, pale yellow, and each contains two nuclei. Aeciospores are carried by wind to young corn plants, where they germinate and penetrate the host tissues. Following infection by aeciospores, red uredospores are formed in pustules on the plant. Uredospores may overwinter in southern and central United States and initiate infections the following spring, thus bypassing the alternate host, woodsorrel, as a link in the life cycle of the fungus.

Uredospores are carried progressively northward by wind currents as the growing season advances and probably furnish the bulk of the initial inoculum of the corn rust in the Corn Belt.

MODERATE TEMPERATURES and frequent heavy dews favor development of common corn rust. Luxuriant and succulent growth of the host, as frequently occurs on soils high in nitrogen, is conducive to rust development.

Little attention has been given to the control of common corn rust because of its minor importance. Inbred lines differ in resistance to common corn rust, but because of the variation in distribution of physiologic races of the rust, the reaction of inbred lines may not be constant when such lines are grown in different localities. Seed treatment and crop rotation have no effect on the disease. Teosinte is the only other host for this rust.

Southern corn rust is found from Massachusetts westward to southern Indiana and southward to the Gulf of Mexico. It has been found in Mexico, Central America, South America, the West Indies, and Africa.

The symptoms resemble those of common corn rust, particularly in the uredial, or summer, stage. The cinnamon-brown pustules tend to be smaller and more circular in outline than those of common rust. Pustules of the telial, or winter, stage are chocolate brown to black and circular to elongate and are distinguished from common corn rust by retention of the epidermis of the leaf over the pustule for a long time.

Puccinia polysora is the fungus that causes southern corn rust. Uredospores are yellowish to golden, globoid to ellipsoid, and finely and sparingly spined. Teliospores, which differ from those of P. sorghi, are angular, irregularly ellipsoid or ovoid, and chestnut brown. Each spore is two-celled and borne on a pedicel one-fourth or less the length of the spore.

No alternate host is known for southern corn rust. Consequently, the spermagonia) and aecial stages have not been described. Much of the initial inoculum in the southern part of the Corn Belt may be carried in by wind currents from warmer areas. Certain species of Tripsacum and silver plume-grass are the only other known hosts of southern corn rust.

Frequent heavy dews favor infection and spread of the disease. Southern corn rust requires somewhat higher temperatures for optimum development than common corn rust. Because of the minor importance of this rust, little attention has been paid to methods of control.

ARNOLD J. ULLSTRUP is a plant pathologist in the Bureau of Plant Industry, Soils, and Agricultural Engineering cooperating with the Purdue University Agricultural Experiment Station.