Studies were made of the effect of temperature and light on the reactions to leaf rust (Puccinia triticina Erikss.) of the differential varieties of wheat used for the identification of physiologic races of this rust. With a large number of races both temperature and light were found to exercise a marked influence. The reactions of all varieties were not, however, influenced in the same direction. With lower temperature, Malakof and Democrat became increasingly susceptible, while Carina, Brevit, and Hussar became increasingly resistant. Webster and Mediterranean did not react consistently in either direction, and Loros was but little influenced by temperature. All of the differential varieties showed a more or less marked tendency to become increasingly resistant under conditions of short day length and weak light. In general, more pronounced changes in reaction were produced by variation of temperature than of light.Surveys for the distribution of physiologic races of leaf rust in Canada were conducted annually since 1931 with the exception of the years 1932 and 1935. Forty-nine races were identified. Most of the prevailing physiologic races were found to be widely distributed throughout the country. Evidence was, however, secured that certain races were largely confined to certain areas. Races 1, 58, 76, and 81 were common for several years in Eastern Canada but were not encountered in the Prairie Provinces until 1940 when one collection of each of the three first-mentioned was made in that area. Races 11 and 53 were largely limited to British Columbia and the adjacent province of Alberta. Races of wheat leaf rust have undergone no marked change in respect to identity or relative prevalence in the last few years in Canada.
