540 Journal of Agricultural Research voi. xix. no. n 



than lumillo (C I 1736), while Acme and D-4 were slightly more resistant 

 than either of the other durum sorts. The emmer varieties were very 

 resistant, Minnesota 1165 being practically immune. 



(6) The Fj generation of crosses between durum and common varieties 

 was as susceptible as the common parent, while F^ crosses between the 

 practically immune emmer parents and susceptible commons were about 

 as resistant as the durum varieties. Thus, in the cross where emmer is one 

 parent, resistance is partially dominant, while in the durum-common 

 cross susceptibility is completely dominant over resistance. 



(7) Each F2 plant which produced viable seed was tested in the F3 

 generation for both rust infection and botanical characters. These F3 notes 

 were used to determine the genotypic nature of individual Fj plants. 

 In the crosses between durum and common in which Marquis was the 

 female parent, 404 Fj plants were tested in the F3 generation and no 

 rust-resistant common wheats were obtained. Likewise, no plants in 

 the F3 generation seemed especially promising for both common wheat 

 characters and rust resistance. In the crosses in which durum was the 

 female, one or two Fj common-headed plants were resistant; but their 

 progeny were worthless from a practical agronomic standpoint. In one 

 F3 family which was grown from a susceptible Fj plant, a number of 

 resistant, vigorous plants were obtained which had common head charac- 

 ters. There is an indication of linkage of durum or emmer characters 

 and rust resistance, since the production of rust-resistant durums or em- 

 mers in the Fj and F3 generations is comparatively easy and the produc- 

 tion of resistant common wheats much more difficult. 



(8) Resistant and susceptible plants obtained either in the Fj or F3 

 generation from crosses of durum and common parents were selected. 

 Resistant and susceptible common, emmer, and durum wheats were 

 available for this study. Greenhouse inoculation studies with a known 

 strain of Pticcinia graminis tritici showed that durum, common, and 

 emmer type plants were obtained in the Fj or F3 generation which 

 were more resistant than the resistant durum parents. Thus, we have 

 transgressive segregation for rust resistance. 



(9) The number of plants available for a study of inheritance between 

 resistant emmer parents and Marquis was not very great. In the F3 

 generation several lax-headed wheats were obtained which had the head 

 shape and naked kernels of common wheats and which were rust-resistant. 

 This shows that rust-resistant common wheats can be obtained by cross- 

 ing susceptible common varieties with resistant emmers. 



(10) The mode of inheritance of rust resistance seems entirely com- 

 parable with the general Mendelian manner of inheritance of botanical 

 and morphological characters. The technic of breeding for rust re- 

 sistance is similar to that of breeding for agronomic characters. 



