BREEDING SNAPDRAGONS 7 



At W'aUhani -^2 varieties of snapdragon were tested for their susceptibility to 

 rust under greenhouse and field cultural conditions. The basis used for designa- 

 tion of the degree of susceptibilit\- was the general quantity of spore pustules 

 present on the infected plants in relation to the destructiveness of the disease. 

 Plants were considered resistant when no spore pustules were visible; lightly rusted 

 when spore pustules present on the foliage were few in number; moderately rusted 

 when pustules were abundant but confined to foliage; heavily rusted when pustules 

 occurred on both foliage and stems, causing wilting of the plants; and severely 

 rusted when pustules were abundant on foliage and stems and resulted in de- 

 struction of the plants. 



In the greenhouse, 16 of these varieties were classified as moderately rusted, 



7 as heavih- rusted, and 9 as severely rusted. In the field tests, all the varieties 

 were severely rusted to such an extent that only a very few of the infected plants 

 survived long enough to permit seed to be harvested. 



To determine whether commercial varieties of snapdragon carried a hereditary 

 factor for res'stance to rust, nine of the varieties found to be very susceptible were 

 interbred with one another. Crosses were made in the greenhouse between Chev- 

 iot Maid and Rose Orange, Cheviot Maid and Laura, Lucky Strike and After- 

 glow, Bronze Queen and Afterglow, Cincinnati and Laura, Lucky Strike and 

 Laura, Rose Queen and Rose Orange, and Afterglow and Cornwallis. 



Progeny from these crosses in the Fi generation were tested under field condi- 

 tions for susceptibility to rust. The population within all these crosses was gen- 

 erally so very susceptible to rust that classification as to degree of susceptibility 

 was not attempted, since possible results were anything but encouraging. How- 

 ever, self-pollinations were made of plants which showed some promise of being 

 capable of surviving the rust sufficiently to produce seed. An F2 progeny from 

 such selections in all the crosses was tested in the field with the result that out of 

 eight crosses, three produced sufificient seed for an F3 generation. These were 

 crosses between Lucky Strike and Afterglow, New Cincinnati and Laura, and 

 Cheviot Maid and Laura, from plants which showed some partial resistance to 

 rust. 



In the F3 progeny tests, two selections out of ten lines from a cross of Lucky 

 Strike and Afterglow yielded plants which showed definite differences in reaction 

 to rust, sufficient to allow classification into resistant and susceptible groups. 

 One selected line had a population count of 104 individuals, of which 33 were 

 susceptible and 71 resistant; the other had a progeny count of 102 plants, of 

 which 67 were susceptible and 35 resistant. Progeny from the other two crosses 

 between commercial varieties were all highly susceptible to rust. Further selec- 

 tions of resistant plants from these two resistant lines of Lucky Strike by After- 

 glow were made for an F4 generation progeny- test. A total population of 1171 

 plants was tested for rust reaction in the field, and 290 individuals proved to be 

 susceptible and 881 resistant. From the data in Table 4 it will be noted that the 

 expected Mendelian ratio of segregation was 878.25 resistant plants to 292.75 

 susceptible, and the observed ratio showed a deviation of only 2.75 plants from 

 the expected. The inheritance of the resistance factor here gives a very good 

 fit for a ratio of 3 resistant plants to 1 susceptible. 



Progen}' selections which were 75 percent or more resistant to rust were re- 

 tained for further breeding work. From 51 resistant plants selfed for an F5 gen- 

 eration, 6 yielded progeny all of which were susceptible to rust; 25 produced 

 progeny that segregated in the ratio of 3 resistant to 1 susceptible individual; 



8 gave progeny which were all resistant; and 12 produced progeny that were less 

 than 75 percent resistant to rust. 



From data presented, it is evident that commercial varieties of snapdragons 

 do carry a hereditary factor for resistance to rust; and in this study, where 10 



