RACE DIFFERENTIATION IN CEREAL RUSTS 71 



decided that a race determination in stripe rust should combine both the 

 field observations and the greenhouse tests. 



Summarizing this section on differential varieties one can name 

 three groups of varieties: 



(1) main differentials (good, proved, universally useful differen- 

 tial varieties) , 



(2) supporting differentials (fairly good varieties, which react 

 parallel to the main differentials but with slight differences, which 

 one day may help to find real differences) , and 



(3) resistant varieties (varieties, so far resistant to all known 

 races, which one day--if susceptible--will immediately show the occurrence 

 of a new race) . 



Nearly all cereal rust race workers dealing with cereal rust have 

 such sets. But there is the modern story about differentials which 

 Dr. Loegering, one of the leading scientists in this field, has already 

 treated in his paper presented at this symposium. This work was mainly 

 done with stem rust of wheat {Pucoinia graminis Pers . f. sp . tvitioi 

 Eriks. $ E. Henn.), and less intensively with stripe rust of wheat. 

 Nevertheless there is some literature on genetic analysis of differential 

 varieties and the efforts to come to single factor lines for stripe rust 

 race determination (Macer, 1966; Allan and Purdy, 1967; Lewellen and Sharp 

 1968; Brown and Sharp, 1969) in recent years. 



The publications quoted last discuss the dependence of susceptibility 

 and resistance reactions on the change in the night-day temperature 

 profiles, leading to the description of "major" and "minor" genes (Sharp 

 and Hehn, 1967), as reported and commented on by Dr. Zadoks in this 

 symposium. One of the demands of Brown and Sharp (1969) is: "researchers 

 working with physiologic race identification in Vuccinia striiformis 

 tritici should avoid use of varieties containing minor recessive genes if 

 at all possible." 



ENVIRONMENTS 



In the everyday struggle of a race determinator , all factors do their 

 best to complicate reproducible results in race determination. Environ- 

 mental effects are well known. The influence of light, humidity, tempera- 

 ture, and nutrition on the different stages of host and pathogen development 

 is described in many papers, and will have to be studied for every host- 

 parasite system, probably in every locality. 



In modern institutions, climate rooms and growth chambers are 

 available and allow standardizing of the methods and should help both 

 cultivation and testing. In our institute, Hille (unpublished) developed 

 a small growth chamber where he was very successful in growing some races 

 of stripe rust. Then we started combined experiments in these growth 

 chambers and in the greenhouse to compare the infection type reactions of 

 some races on some differentials. I expected the good differentials 

 would react the same in both systems, since they had worked very well in 

 the climate greenhouse with its relatively unsteady conditions, and that 

 unstable or bad differentials would exhibit clear variation in reaction 

 types in the growth chambers. Here are some of the results. 



