164 TORREYA 



being responsible for a particular disease. Remarkable progress has been made 

 in the study of the nature of the viruses. Duggar, Kunkel, Smith, Stanley, and 

 many others have made important contributions. 



6. Disease resistance. From the earliest times it was observed that 

 species and varieties varied in their susceptibility to disease, and the resis- 

 tant ones were selected in order to minimize loss. In recent years great progress 

 in the selection of these has been made and programs have been developed in 

 the field of plant breeding for combining the resistant quality with other desir- 

 able characters. Success is dependent upon the close cooperation of the plant 

 breeder and the pathologist. 



Orton (1899 and later) stressed the value of types of watermelons resis- 

 tant to the wilt disease and by 1913 had developed commercial varieties. Norton 

 (1910) obtained varieties of asparagus resistant to the rust. Jones and Oilman 

 (1915) began their work on cabbage resistant to yellows. Edgerton (1918) 

 and Pritchard (1922) have developed wilt-resistant tomatoes. Jagger and Scott 

 (1937) obtained cantaloupe varieties resistant to the powdery mildew. 



Finding resistant stock is the first step in any breeding work. The species 

 or varieties may be brought in from other countries and used in the program. 

 Wild potatoes have been sought in Mexico and Peru, and melons from India 

 have proved useful. Graves is finding chestnuts from the Orient useful in devel- 

 oping hybrids of our native chestnut which are resistant to blight. Barley, oat, 

 and wheat varieties have been carried from one part of the world to another 

 and serve as basic stock in breeding programs. 



In most groups of economic plants, studies on varietal resistance have been 

 made, for example: Reed, Griffiths and Briggs (1925) on the resistance of oat 

 varieties to both loose and covered smuts, Reed and Melchers (1925) on the 

 resistance of sorghum varieties to the covered smut, and Tisdale et al. (1923) 

 on the resistance of varieties of wheat to the flag smut, and (in 1925) to bunt. 

 At the Institut fiir Pflanzenbauund Pflanzenziichtung, Halle-Saale, students 

 of Director Th. Roemer have made similar studies of several of the cereal smuts. 



Rieman (1939) stated that about 80 resistant varieties of vegetable crops 

 had been developed and at least 20 of these were recognized by the trade, in- 

 cluding asparagus resistant to rust, snap beans to mosaic, cabbage to yellows, 

 corn to Stewart's bacterial disease, lettuce to brown blight and powdery mil- 

 dew, peas and tomatoes to fusarium wilt. Coons (1937) estimated that about 

 one-quarter of the acreage devoted to 17 important crops in the United States 

 was planted to disease-resistant varieties. 



Breeding for disease resistance is a difficult and time-consuming procedure 

 and there are many hazards by the way. Frequently new physiologic races of 

 the pathogen appear. This is well illustrated in potato breeding for blight re- 

 sistance. The first attempts to obtain resistant varieties were made in the late 



