334 GENETICS OF FUNGI 



are dicarvotic. The thousands of nuclear divisions that occur 

 meanwhile are conjugate, and the complete cycle from the mono- 

 caryotic to the dicarvotic condition and back again to the mono- 

 caryotic may require a period of, at one extreme, only a few days 

 to, at the other, 5 to 7 years, as in Cronartium ribicola. 



Inability to grow rusts on artificial media has no doubt inter- 

 fered to some extent with genetical studies of them. Nevertheless 

 such studies have been energetically pursued, especially by Stak- 

 man and his associates at the Minnesota Experiment Station and 

 by a ^roup at tne Dominion Cereal Rust Laboratory in Canada. 

 The presence of barberry bushes in areas devoted to cereal crops 

 permits the development in nature of new races or strains of rusts 

 by hybridization. There is abundant evidence that such new- 

 hybrid rusts are continuously beino- developed and that their 

 presence accounts for the breaking down of resistance in cereal 

 varieties that possess a high degree of resistance to old strains of 

 rusts. The production of new strains of rusts tends to nullify the 

 laborious efforts of plant breeders to develop resistant varieties of 

 cereals and to control cereal rusts bv use of these resistant varieties. 



Newton and Johnson (1940) crossed Puccini a graminis tritici 

 and P. graminis avenae, finding them completely interfertile. 

 These workers were concerned primarily with pathogenic po- 

 tentialities. Crosses within the avenae variety showed that the 

 less virulent pustule type is dominant over the more virulent type, 

 whereas within the tritici variety the less virulent type is dominant 

 in some crosses but recessive in others. In reciprocal crosses be- 

 tween these two varieties the maternal cytoplasm appears to exer- 

 cise the controlling influence. A cross between a variant whose 

 urediniospores were orange and a variant whose urediniospores 

 were grayish-brown gave all normal red color in the Fi genera- 

 tion. When selfed, four different colors-red, orange, grayish- 

 brown, and white— appeared in the F 2 generation, with a distribu- 

 tion ratio suggesting 9:3:3:1, respectively. 



In experiments with physiologic races of P. graminis tritici, 

 Johnson and Newton (1940) found that absence of pustules, 

 which they called O type, was dominant over large pustules (4 

 type) on Kanred wheat. When this hybrid was selfed, the O type 

 was approximately 3 times as abundant in the F 2 generation as 

 the 4 type. In this instance pathogenic behavior is governed by 



