6 MASS. EXPERIMENT STATION BULLETIN 400 



and seed pods of the plants. The progeny from the hybrids was grouped either 

 as resistant or susceptible, with the term resistant used in the sense that no spore 

 pustules were observed on any portion of the plants. Frequently in resistant 

 selections an inhibited or modified type of rust infection occurred on plants, 

 evidenced by flecking or chlorotic pattern on the leaves. What apparently occurs 

 in this type of resistance to rust is that invasion of the leaf cells takes place, re- 

 sulting in a partial breakdown of the chloroplasts or cells and producing a mild 

 type of chlorosis; but further destruction of the cell tissues is prevented by some 

 physiological reaction between the host cells and the rust fungus. Mains and 

 Jackson (8) in their studies of physiologic forms of leaf rust in wheat noted a 

 fleck type of resistance to wheat rust which appears to be similar to that observed 

 in snapdragons. 



The Waltham Field Station hybrids, developed by interbreeding the Mains' 

 strains of snapdragons with commercial varieties, have proved highly resistant 

 to rust and are a definite improvement over the original resistant parent plants 

 from which they were bred. Flower colors in this group have been limited to 

 white, yellow, and various shades of pink. Bronze-colored flowers of a good shade 

 have been difficult to obtain in these hybrids. Selection and inbreeding of the 

 resistant lines have been continued to develop pure-breeding seed stock lines, in 

 the course of which process it has been observed that certain characteristics of 

 the wild species of Antirrhinum are frequently inherited, such as smaller flowers 

 than those of commercial varieties and a variability in seeding capacity of the 

 plants. Backcrossing the resistant strains to the original susceptible parents 

 and reselection have lessened these traits to a considerable degree. Lines selfed 

 for four to five or more generations are generally lower in hybrid vigor than 

 material frequently crossbred. 



Three of the Waltham Field Station resistant strains of snapdragons were 

 included in trials in California by Blodgett and Mehlqu'st (2), who tested the 

 reaction to rust of some 37 strains in 11 different localities withm the seed-grow- 

 ing area of that State. Performance of the three Waltham strains was on the 

 average as good as that of the other strains, based on the scale of rust resistance 

 used by the California workers. One interesting feature brought out in the Cali- 

 fornia tests IS the variability of rust resistance in the same lines under similar 

 climatic conditions. At Guadalupe all the strains of snapdragons tested were 

 highly susceptible to rust disease; but in the other ten localities where the tests 

 were conducted, these same strains, with few exceptions, showed definite resistance. 



Development of Rust-Resistant Strains by Interbreeding 

 Susceptible Commercial Varieties 



A number of workers have investigated and reported on the reaction of com- 

 mercial varieties of snapdragon to rust disease. Peltier (10) studied the resistance 

 to rust of some 40 varieties of snapdragon and concluded that all of them were 

 equally susceptible. Doran (3), who tested 46 varieties of snapdragon for their 

 rust reaction, reported that no variety was completely resistant but that some 

 were relatively resistant. Mains (9), using some of the same varieties that Doran 

 tested, found them all very susceptible, with no pronounced differences between 

 commercial varieties; but he did note individual plant differences in reaction to 

 rust within a variety. He self-pollinated those plants showing greater resistance 

 to rust for several generations and was able to obtain highly rust-resistant types, 

 but as inbreeding continued he encountered self-sterility that interfered with 

 selection of homozygous lines. Emsweller and Jones (4) report that their attempts 

 to find rust-resistant snapdragon plants in commercial plantings in the seed pro- 

 duction areas of California were unsuccessful. 



