[newton] wheat stem RUST 161 



type and degree of infection the symbols adopted by Stakman and 

 Levine in their work on biologic specialization were used. A detailed 

 explanation of the meaning of these is given in Table II. Briefly 

 stated, 0, 1 and 2 designate varying degrees of resistance, collectively 

 referred to by the symbol R in the "Key to Biologic Forms" given 

 above. On the other hand, 3 and 4 designate degrees of susceptibility, 

 referred to in the key by the symbol S. Experience has shown that 

 the degree of resistance or susceptibility of a given wheat variety to 

 a given biologic form, under greenhouse conditions, is remarkably 

 constant. In the field, however, there is a tendency towards a some- 

 what lighter degree of infection. In some cases, a variety that would 

 ordinarily score 3 in the greenhouse may score 2 in the field. Varieties 

 are given the score representing the degree of infection to which they 

 most nearly approximate, this figure being followed, where necessary, 

 by plus or minus signs to indicate deviations from the standard. 



The procedure just outlined for identification of the forms was 

 facilitated by the use of the accompanying key devised by Stakman 

 and Levine. This key is constructed upon the same principle as an 

 ordinary botanical key. The compound symbols employed consist 

 of the abbreviated designations for the differential hosts, followed by 

 R or S, denoting resistant and susceptible, respectively. The use 

 of the key will be illustrated by the following example: A form 

 collected on Kanred at Saskatoon, Saskatchewan, on September 14, 

 1920, was cultured on the hosts given in Table I. Marquis proved 

 susceptible. This threw the form in question into the second main 

 section of the key, viz., Ma— S. The test on Kanred showed this 

 variety also to be susceptible. This led us to section KRd^ — -S of the 

 key. Kota also proved susceptible. This placed the form in Ko — -S. 

 As Arnautka and Mindum were both resistant, we arrived at Mnd — R. 

 Kubanka, on the other hand, was susceptible, thus bringing up to 

 Kub- — S. But one variety, Acme, remained to be examined. Since 

 this proved susceptible the form was identified as number XVI 1 1. 



Results 



Fourteen biologic forms have been demonstrated, by the methods 

 described, to be present in Canada. These forms have proved to be 

 identical with some of those described by Stakman in the United 

 States. In the latter country, however, a considerable number of 

 additional forms have been demonstrated.^ The date and place of 

 collection, host on which collected, and complete infectional char- 

 acterization of the Canadian forms are summarized in Table III. 



^Unpublished data from E. C. Stakman. 

 11— E 



