70 STAKMAN 



A system was devised for identifying races by their parasitic effects on a 

 standard set of twelve varieties that seemed to be representative of the 

 principal types of wheat. The races are designated by number. Each year, 

 races are identified from about a thousand collections of rusted wheat from 

 the principal wheat-growing areas of the United States and Mexico. Similar 

 studies are made in Canada, so that the geographic distribution and popu- 

 lation trends of the principal races in North America during the past thirty- 

 five years are fairly well known. What has been their record? 



Following the terrible epidemic of 1916, rust-resistant durum wheats, 

 from which macaroni is made, were substituted extensively for bread wheats 

 in the spring-wheat region of the United States, but within a few years rust 

 races appeared to which they were completely susceptible. Kanred, a good 

 winter wheat selected in Kansas, appeared to be immune from rust, but 

 almost as soon as it was distributed rust races were found to which it is 

 completely susceptible. In 1926 the rust-resistant hybrid variety Ceres, a 

 good spring wheat, was distributed and soon became by far the most popu- 

 lar variety in the spring-wheat region of Minnesota, the Dakotas, eastern 

 Montana, and adjacent Canada. Ceres seemed to have solved the stem rust 

 problem in spring wheat. But in 1935 it was suddenly and tragically ruined 

 in a terrific epidemic caused principally by race 56 of stem rust. This race 

 had first been isolated from barberry in Iowa in 1928, two years after Ceres 

 was first distributed, and by 1934 it was the most prevalent race in the 

 United States. In 1935 race 56 almost literally exploded and virtually ended 

 the career of Ceres wheat. Many other varieties, however, had been in the 

 making, among them Thatcher, which resulted from crosses involving three 

 parents. It had been severely tested against all known races of stem rust 

 prior to its first distribution in 1934. It came unscathed through the epidemic 

 of 1935 and another severe epidemic in 1937. It was extremely susceptible 

 to orange leaf rust and to Fusarial head blight, however, so that it could not 

 be grown successfully in the more humid parts of the spring-wheat area where 

 these diseases are most prevalent and destructive. It persisted, however, in 

 the drier areas of western Canada, where it still produces well on millions of 

 acres. 



Another series of varieties were soon released that had the "Hope type" of 

 resistance. The variety Hope resulted from a cross between Jaroslav emmer, 

 which had no virtues except rust resistance, and Marquis wheat, which had 

 all the virtues except rust resistance. Hope did not have the qualities needed 

 in a commercial bread wheat, but it was widely and successfully hybridized 

 with good bread wheats. Similarly, highly resistant durums had been pro- 

 duced by crossing good macaroni varieties with Vernal emmer. Thus the era 

 of "Hope resistance" began. Again it looked as if stem rust was under con- 

 trol in the spring wheat area. 



From 1938 to 1949, inclusive, there were no epidemics of stem rust in the 



