| Summary: | <p>Stem rust of wheat <u>Puccinia graminis tritici</u> Eriks. and Henn., has always been a limiting factor in the production of wheat in Western Canada. According to Hanna (12), rust damage was first reported in 1891 and the first attack of epidemic intensity occurred in 1904. Other severe epidemics of stem rust accurred in 1916, 1923, 1927, 1935, 1938 and 1953. Attacks little short of epidemic intensity accured in 1919, 1921, 1925, 1930 and 1954. Greaney (11), estimated that during the 11 year period of 1925-1935, the average annual loss due to reduction in yield was approximately $30,000,000. In addition, $2,000,000 was lost annually due to lowering of grade of the wheat. </p>
<p>After 1939, resistant varieties were widely grown in Western Canada. Craigie (8) estimated that in the 5 year period of 1939-1943, the average annual increase in farm income resulting from the growing of rust resistant varieties in Manitoba and Eastern Saskatchewan was approximately $27,000,000.</p>
<p>Unfortunately, the varieties grown in Western Canada after 1939, were not resistant to all races of stem rust. Races which were considered unimportant, but, could attack the common varieties, were relieved of competition with the prevalent races and when environmental conditions were favorable, reached epidemic proportions. In addition, new, virulent races, hybridized on the common barberry, also had an opportunity to develop on the new varieties and become prevalent. Race 15B is a striking example of such a change.</p>
<p>According to Stakman (30), race 15B has been known to exist on the barberry since 1939, but was never abundant until 1950 when it became prevalent on previously resistant varieties of bread wheat and durum. Durums, in particular, have been severely rusted. Rodenhiser and Moore (28) mention that 10,000,000 bushels of durum wheat, which was about one-fifth of the crop, was lost in the United States in 1951. In Western Canada, race 15B was first reported in 1946 and became widely distributed by 1951. Hanna (12), reported that three distinct strains of race 15B have been differentiated 15B-1, 15B-2 and 15B-3. Strain 15B-2 which has recently been renamed 15B-4, is a specially virulent strain in durums and is rapidly increasing in prevalence.</p>
<p>The wheat stem rust situation in Western Canada was relatively stable from 1938 to 1949. During this time the durum varieties commercially grown were resistant to the prevalent races of stem rust. The varieties, Carleton and Stewart, which derived their resistance from Vernal emmer, were extensively grown. </p>
<p>After 1950 and the increase of race 15B, all commercially grown durum varieties were severely attacked. The epidemics of 1953 and 1954 almost completely destroyed the durum crops grown in Western Canada. As a result, the acreage of durum production was greatly reduced in following years and the area of production shifted westward, out of the main rust area of Manitoba and Eastern Saskatchewan. </p>
<p>The need for durum varieties resistant to race 15B is apparent. In order to develop these varieties sources of resistance to this race must be available. One way of obtaining these sources is to make a collection of local and foreign varieties and to test their reaction under epidemic conditions of race 15B. Valuable work in testing of varieties for reaction to stem rust is done in the International Rust Nursery. Varieties from all parts of the world are introduced and tested in the nursery for their mature plant resistance to many races of stem rust. A report is issued every year describing the performances of the durum introductions in the nursery. Many of the varieties have been found to be almost immune to race 15B, but usually possess many undesirable agronomic characteristics.</p>
<p>The purpose of this study was to determine the inheritance of resistance to race 15B of a number of durum varieties obtained from the International Rust Nursery.</p>
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