66 EVA FUCHS 



Race classification or taxonomy is in a constant state of flux. 

 Practical breeding objectives, which may change at any time, have 

 to be combined with internationality or universality in this 

 "small world". 



INTRODUCTION 



To begin with the truth, I am just an old fashioned, patient (too 

 patient, perhaps), practical determinator of races of stripe rust of 

 wheat (Puceinia striiformis West.), working mainly with seedling plants 

 in the first leaf stage. I am going to show you an everyday struggle 

 with the business of determining physiologic races of stripe rust. In 

 doing this I should give you a brief introduction to this host-parasite 

 couple. We are dealing with an annual host (exhibiting a great deal of 

 variability grown under conditions that allow rapid changing of 

 varieties) . Also, the alternate host for P. striiformis is unknown. 

 Both points are different from your problems with tree rusts, but there 

 should be some room for comparison. 



Race determination in cereal rusts is generally based on differen- 

 tial varieties of the uredial host tested under more or less controlled 

 conditions and distinguished by different "infection types" of the host- 

 parasite system. The infection types of the wheat-stripe rust system 

 have been defined as follows: 



i = immune, no kind of fleck or pustule visible 



i-0 = some light chlorotic flecks ("oo" with other authors) 



= chlorosis and necrosis, no pustules 



1 = chlorosis and necrosis, very few and very small pustules 



II = chlorosis, less necrosis, few pustules, small to medium 



size 



III = chlorosis, abundant pustules of normal (susceptible) size 



IV = no chlorosis, abundant normal pustules 



My work with this host -parasite system has led me to make three 



generalizations about determination of rust races. First, the discovery 



of a good differential variety is mainly luck. Second, a list of races 



is an agreement (convention) , and third, the practicability of a race 

 list is a kind of historical question. 



DIFFERENTIAL VARIETIES 



Physiologic races were first detected (and still will be) when one 

 variety was highly infected in one region but not in the other, while a 

 second variety reacted in an opposite manner. "Region" may be area or 

 time. All classical differential sets started this way. Later on they 

 were refined, enlarged, and sometimes diminished again. With the first 

 ecstasy of discovery Gassner and Straib (1932) described 14 races with 

 10 differentials. When Straib left Braunschweig in 1945 he had defined 

 54 races using 11 differentials. After 14 years on the job, I could not 

 agree that some of the original differentials were of value. So, I com- 

 bined some races which showed differences only on the unreliable differ- 

 ential varieties (Fuchs, 1960, 1965). 



