QUESTIONS OF RACE DIFFERENTIATION WORK IN CEREAL RUSTS 



Eva Fuchs 



Biologische Bundesanstalt fur Land- und Forstwirtschaft 3 



Braunschweig, Germany 



ABSTRACT 



Out of practical work with physiologic races of the stripe rust 

 of wheat and barley (?uccinia striiformis) -some general and 

 some especial aspects of differentiation and description will 

 be discussed. 



Race determination in cereal rusts is based on differential 

 varieties of the uredo-host, tested under more or less controlled 

 conditions, and distinguished by different infection types of 

 the host-pathogen-system. 



The detection and the replacement of differential varieties has 

 a long history and has during years and a generation of scien- 

 tists shifted from' empirically identified differentiating 

 varieties to genetically labeled ones. 



Three main groups of differential varieties are generally 

 employed: main differentials, additional differentials and 

 screening varieties. 



The question of internationality of differential sets will be 

 touched. 



Host and pathogen are very much dependent on the environmental 

 conditions. Many efforts have been made to standardize the 

 growth conditions of both and to reach reliable and reproducible 

 results . 



In certain cases the determination of physiologic races, in the 

 seedling stage of the host, under the rather artificial condi- 

 tions holding in greenhouse and growth chamber is not sufficient. 

 The facts (1) that the seedling cereal plant generally is more 

 susceptible than the same plant in the later stages of develop- 

 ment, and (2) that the plant throughout its various growth stages 

 is able to show wider differentiation of rust infection (both in 

 type and degree) , together with numerous observations with the 

 same set of varieties in different places and different years, 

 has led to the description of "field races". 



Some additional characteristics in race determination can be the 

 differences in "transfer-time", the differences in optimal 

 temperatures, and the differences in germ tube development of 

 the spores . 



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