Stem rust came again into the picture with races 6, 7, and 8, to which most of the new varieties are susceptible. Race 8 is widespread and abundant. Race 7 appears to be rapidly increasing. A new, more virulent biotype, 7A, has been reported from Canada. Race 6 occurs infrequently.

Segregates from crosses designed to combine resistance to those races with resistance to other races of stem rust, crown rust, and the helminthosporium blight were in the yield-testing stage in 1953.

RUSTS OF BARLEY were not generally considered important enough to justify intensive breeding for resistance to them until about 1935, when many fields in the North Central States and western prairie regions were severely damaged by stem rust. It is now generally recognized that leaf rust may do damage generally in all humid barley-growing areas but principally on winter barley in the Eastern and Southeastern States. Resistance to both rusts has been an important objective in most barley-breeding programs in those areas.

The most important commercial variety resistant to stem rust was found by a farmer, S. T. Lykkens of Kindred, N. Dak. In 1935 he decided to plow up his field of Wisconsin Pedigree 37 because of severe damage by stem rust. He observed a single plant that was free of rust, saved the seed, planted it in his garden the next spring, and in subsequent years sold seed to his neighbors. By 1942 it was a recognized commercial variety. Its resistance has since been verified by others. It has been included in numerous yield trials and tests of quality. Although it has some defects, such as weak straw, it has compared favorably with other varieties in most important respects. It has been named Kindred and is now grown more extensively for use in malting than any other variety in the principal barley-producing area of the United States.

Other commercially grown varieties resistant to stem rust are Peatland, Mars, Plains, Feebar, and Moore.

Fifty or more other varieties, most of them from the world collection of barley from various foreign countries and as selections from farmers' fields known to be resistant to stem rust, are valuable sources of resistance for use by plant breeders, even though they are not grown commercially. Progress has been made also in breeding varieties resistant to leaf rust, but only one, Goliad, had been released in 1953. Goliad is also resistant to stem rust.

THE LARGE NUMBER of physiologic races (240 or more of stem rust of wheat, for example) has sometimes seemed an insurmountable obstacle to the successful breeding of resistant varieties. Nevertheless a great deal of progress has been made. From the years of experience has come a better understanding of their relation to breeding programs.

Our concept of a large number of races stems mostly from the manner in which they are identified. Because of technical difficulties, it is not generally feasible to determine the ability of each of a large number of collections of rust spores to infect each of a large number of varieties in an advanced stage of growth in the field. Yet this is the information that is needed. The nearest approach to this so far devised for identifying races is to infect young plants, usually seedlings, growing in a greenhouse. Only a few varieties, often not more than a dozen, are included in the tests, and they are usually the same year after year. They are known as host testers. It is hardly to be expected that results from the tests could be applied directly to breeding without due consideration of the conditions under which they are obtained.

One important fact is that resistance or susceptibility in the seedling stage in the greenhouse does not necessarily mean resistance or susceptibility when a plant is in an advanced stage of development in the field. A physiologic race that infects a given variety in the seedling stage but not when the plants are approaching maturity is not likely to be a serious menace to that variety, but it may add to the number of physiologic races.

Another fact that makes breeding for resistance to all prevalent races less formidable than it may seem to be is that many varieties are resistant to several races. It is known, for example, that Kanred in the seedling stage is resistant to 11 races of stem rust. Hope is resistant to at least 17 races, Red Egyptian to 26, Kenya-Gular to 30, and certain other Kenya derivatives to at least 35. They may be and probably are resistant to a larger number in advanced stages of growth in the field, although, as we indicated, the exact situation would be hard to determine.

Somewhat the same applies to leaf rust of wheat and to the rusts of oats and barley. In most cases it is possible on the basis of known information to choose a small number of varieties of each crop that collectively are resistant to all known races of a given rust. If this resistance were to be concentrated in a single variety, and no new races appeared, the job of producing resistant varieties would be finished except as there might be need for resistant varieties for other areas or for other reasons. And this, of course, is what cereal breeders for rust resistance have been trying to do for some 75 years with considerable success.

From this point of view, the appearance of a new physiologic race does not mean defeat. Rather, it should be regarded as a warning of danger: A highway sign is designed not to frighten but to be respected.

JOHN H. MARTIN is an agronomist in charge of sorghum investigations in the division of cereal crops and diseases at the Plant Industry Station, Beltsville, Md. He has written many bulletins, circulars, and other publications on cereal culture.

S. C. SALMON is an agronomist in charge of investigations of wheat in the same division. Before he joined the Department of Agriculture in 1930 he was on the staff of Kansas State College.