8 MASS. EXPERIMENT STATION BULLETIN 400 



varieties were interbred, only 2 produced progeny carrying a relatively h'gh degree 

 of resistance. The nature of the resistance to rust in the commercial hybrids is 

 different in its mode of expression from that of hybrids obtained from Mains' 

 strains. The inheritance of resistance to rust in the Mains' strains was observed 

 to be governed by a simple dominant Mendelian factor. While the resistant 

 lines selected from the progeny of commercial varieties have not been interbred 

 sufficiently to determine their degree of heterozygous or homozygous condition, 

 the data available indicate that the hereditary factor for resistance to rust is 

 either a recessive or a modified dominant type. Some evidence which would 

 indicate presence of modifying factors was the occurrence of Antirrhinum species 

 characters in progeny of resistant selections. Plants showing species characters 

 were relatively more resistant to rust than those bearing less of these character- 

 istics. Emsweller and Jones (4) observed the presence of plants susceptible to 

 rust in presumably homozygous lines of snapdragon, and intimate that modifying 

 factors for resistance to rust may be present in commercial varieties. The possi- 

 bilit}' of disease escape and environmental conditions having some influence on 

 variation in susceptibility of resistant plants has been considered; but the fact 

 that plants produced by cuttings from resistant mother plants have remained 

 free from rust for three years in field tests is considered conclusive evidence 

 that differences in rust reaction, noted in the resistant hybrids developed from 

 susceptible commercial varieties, are due to hereditary factors. The resistance 

 and susceptibility of different hybrid strains of snapdragon to rust were most 

 effectively demonstrated under field tests as illustrated in Figures 1 and 2. 



Table 4. Rust Reaction of Progeny from Selected Resistant Lines 



Obtained by Intercrossing Susceptible Varieties of Snapdragons 



(Lucky Strike X Afterglow). F4 Generation. 



Progress in Combining Rust Resistance witli a Desirable Plant Type 



A good selection of flower colors in light and dark pinks, yellows, bronzes, and 

 white has been developed in the Field Station rust-resistant strains of snap- 

 dragon. Some strains are 80 to 90 percent resistant to rust, and in form of flower 

 and habit of plant growth are comparable to commercial strains. They are good 

 seed producers and bloom well in the winter under greenhouse conditions. The 

 most promising strains are those developed from intercrossing susceptible com- 

 mercial greenhouse forcing varieties. Resistance to other diseases, such as ver- 

 ticillium wilt and powdery mildew, has not been conclusively ascertained although 



