[newton] wheat stem RUST 191 



spore stage. They contribute largely to the general dissemination of 

 the disease throughout the season. Certain grasses may even harbour 

 special biologic forms. 



Since nothing was known concerning the reaction of most of the 

 biologic forms on grasses, 29 species were inoculated with 3 of the 

 most prevalent and diverse forms of rust found in Canada, IX, 

 XVII, and XVIII. The results are given in Table V, and show 

 that there are only slight differences in the infection capabilities of 

 these 3 forms towards the grasses tested. This is in sharp contrast 

 to the behaviour of the same biologic forms on the 12 wheat varieties 

 used as differential hosts. Of course, it must be borne in mind that 

 this list of differential hosts was arrived at only after much experi- 

 mentation. In an early stage of her investigations, the author tested 

 120 varieties of bread wheats with 6 collections of wheat rust without 

 finding any evidence of biologic specialization. It is therefore quite 

 possible that further work may discover differential hosts also among 

 the wild grasses. 



Discussion of the Principal Issues 



The study of biologic forms of the pathogène causing wheat stem 

 rust {Puccinia graminis triticis) in Canada suggests that climate is 

 not a controlling factor in the distribution of these forms. The 14 

 forms collected in various parts of Western Canada proved to be , 

 identical with forms isolated by Stakman and Levine in collections 

 made from widely separated points in both northern and southern 

 United States. This was rather interesting as, before carrying out 

 this experiment, it was thought that rust found in the protected foot- 

 hills of the Rockies and in northwestern Alberta might be quite 

 different from that found in the open plains of the Red River Valley 

 of either Canada or the United States. 



In connection with these studies, consideration was given to the 

 old problem of the seasonal spread of rust from south to north. In 

 case the rust moved northward in waves across the continent, and 

 the biologic forms varied in point of origin, then it would be expected 

 that they would appear at successive dates during the summer, varying 

 with the remoteness of the point of origin. Accordingly, the place 

 and date of collection of each form were carefully noted. Although 

 in the three observed years no definite succession of biologic forms 

 was found, yet it was interesting to note that the same biologic form, 

 XVII, appeared first each year, having been collected as early as July 

 5, and a form attacking emmer heavily, IX, was always one of the 



