BREEDING SNAPDRAGONS 



,5 



Progeny resulting from reciprocal crosses between the No. 294 strain and White 

 Rock and Ceylon Court, in the Fi generation, segregated in a ratio of 50 percent 

 rusted to 50 percent resistant plants. 



Selections of the most resistant plants in the F2 generation from the cross 

 between No. 294 and White Rock were inbred, and from a population of 557 

 plants 417 were resistant to rust and 140 were rusted. The F2 generation from 

 crosses between No. 294 and Ceylon Court produced 509 individuals of which 382 

 were rust-resistant and 127 were rusted. Data on progeny segregation for resis- 

 tance, when subjected to statistical analysis for goodness of fit, show that calculated 

 and actual segregation ratios closely approached a ratio of 3 rust-resistant plants 

 to 1 rust-suscept'ble (Table 3). On the basis of such data, it is concluded that 

 resistance to rust disease is inherited as a dominant factor, as previously noted 

 by White (13) and Mains (9). 



Table 3. Progeny Segregation for Resistance to Rust in Crosses 

 Between Resistant Strain No. 294 and Susceptible Varieties. 

 f2 generation 



In the F3 generation, selected rust-resistant progeny varied in degree of re- 

 sistance depending upon the individual heredity of the lines inbred: a few con- 

 tinued to breed true for rust resistance, being 80 to 90 percent resistant; others 

 were highly susceptible to rust; and many segregated again in a r^tio of 75 percent 

 resistant to 25 percent susceptible plants. Determinations of the degree of rust 

 resistance of the hybrids by exact progeny counts were impossible, since manj- of 

 the individuals in selected lines were killed by wilt disease ( VerticilUum albo-atrum) 

 before infection by rust occurred or the rust disease reached its peak in the field. 

 In the early phases of the work, restricted greenhouse area prevented the use of 

 pot culture and artificial inoculation methods which have been found to be one 

 means of limitmg these other disease factors encountered in the field. However, 

 as in many cases of adversity, the introduction of the wilt disease factor has not 

 been without value, as it provided conditions for observing the reactions of 

 rust-susceptible varieties and resistant strains to the wilt disease. 



Rust symptoms on individual plants within susceptible and resistant strains 

 were variable, as evidenced by the number and size of spore pustules formed on 

 the plants. No attempt was made to classify very finely the degree of suscepti- 

 bility of the individuals into distinct groups even though there were marked 

 differences in the distribution of spore pustules (uredinia) on the leaves, stems, 



