BREEDING SNAPDRAGONS 11! 



Physiologic Races of Snapdragon Rust 



The recurrence of rust disease on rust-resistant strains of snapdragon in the 

 coastal regions of California in 1936 has been attributed to the appearance of a 

 second strain or form of the rust fungus. Yarwood (14) reports that a second race 

 of rust was isolated from infected rust-susceptible snapdragon plants and desig- 

 nated it as physiologic race 2 to distinguish it from the more virulent form which 

 he calls physiologic race 1. The differentiation of the physiologic forms of rust 

 reported wbs by use of excised leaves and apparently with unpurified rust cul- 

 tures. At Waltham no physiologic strains of the rust fungus have been observed 

 although tests were made for the presence of mutant forms by the use of excised 

 leaves and direct inoculation of resistant plants with spores from plants showing 

 partial and severe infection. No rust has appeared on plants grown from cut- 

 tings taken from rust-free plants over a period of three years. Presumably such 

 plants should be ideal material for differentiation of any mutant forms of the 

 rust that might occur. 



Resistance of Field Station Snapdragons to Other Diseases 



Snapdragon plants for breeding work have been planted in a field where plants 

 infected with rust and wilt fungus ( Verticillium albo-atrum) were plowed into the 

 soil each year for a period of four or five years. The wilt disease has been quite 

 destructive to the plantings each year, its virulence varying with seasonal con- 

 ditions. 



In some seasons rust-resistant strains appeared to be less affected by the wilt 

 disease than were rust-susceptible varieties of snapdragons. Plant counts for 

 susceptibility to wilt have been made each year but the results were variable. 

 Certain strains were much more severely affected by the disease in some seasons 

 than in others. Wilting caused by severe infestation of rust made differentiation 

 between plants killed by rust and those killed by Verticillium rather difficult — 

 especially since both were frequently present on the same plants — even though 

 inspection for distinctive vascular discoloration was resorted to as a means of 

 identification of wilt infection. 



In the greenhouse, powdery mildew attacks the leaves and stems of snapdragon 

 plants each spring and frequently is virulent enough to be very destructive. Mild 

 infections of powdery mildew have been present on Field Station rust-resistant 

 strains in the greenhouse, but usually the plantings ?re removed in the spring 

 before the disease becomes sufficiently distributed through the different strains 

 to permit complete observations to be made. Commercial varieties of snapdragon 

 have not been observed to show any differences in susceptibility to mildew under 

 natural conditions for infection. 



Stem and leaf blight caused by anthracnose {Colletotrichum Antirrhini Stew.) 

 is generally prevalent under field conditions. This disease is even more destruc- 

 tive to snapdragons than rust in some seasons and, while it usually occurs in the 

 late fall, frequently affects seed production. So far, no differences in susceptibility 

 to the disease have been observed in any of the commercial varieties or the rust- 

 resistant strains. 



Rust Reaction of Commercial Resistant Garden Varieties of Snapdragon 



In 1934 seedsmen of California introduced and gave considerable publicity 

 to a number of rust-resistant garden snapdragons. These new strains originated 

 from some of Mains' strains which had been sent to the California Agricultural 

 Experiment Station. From such material, Emsweller and Jones (4) successfully 

 bred acceptable, improved types that were later released to seedsmen for further 



