Griffee: Rust Resistant Oats 



189 





SEGREGATION OF THE PROGENY OF THE RESISTANT SECOND 

 GENERATION PLANTS 



Figure 21. In order to separate the pure resistant (homozygous) plants of the second genera- 

 tion from those that carry susceptibiHty recessively (Figure 20), a few seeds of each resistant 

 second generation plant are planted in the greenhouse. These seedlings when only a few weeks 

 old are inoculated with rust spores. Some families are found to be entirely resistant, but in others 

 rusted individuals are found. The pure, resistant families are planted in the field, while the others 

 are discarded. The greenhouse test reduces the number of lines to be grown to one-third what 

 would otherwise be necessary, and homozygous resistant plants are obtained for field selection a 

 year earlier. (See text, p. 188.) 



In making seedling inoculations, two 

 methods were used. The first was the 

 ordinary hand inoculation method. 

 This consists of first wetting the seed- 

 ling and then spreading uredospores 

 on the lower surface of the seedling leaf 

 with a small scalpel. Seedlings are 



then placed in an incubation chamber 

 for 48 hours. The second was the 

 brushing method'^ used for inoculating 

 plants to produce a quantity of inocu- 

 lum. In this the seedlings are sprayed 

 with water and then brushed lightly 

 with rusted seedlings. The incubation 



Table I. Numbers of F3 Oat Families Segregating or Breeding True for Resist- 

 ance to Stem Rust, and a Comparison of Obtained and Expected Ratios 



Table II. Numbers of resistant and susceptible Seedlings of All Fz Families Segre- 

 gating for Resistance and Susceptibility to Stem Rust, and Comparison of 

 Obtained with Expected Ratios 



* P'irst used by O. S. Aamodt at the Minnesota station in rust inoculations with wheat seedlings. 



