DISEASE RESISTANCE IN ANNUAL CROPS 59 



Table 2. Composite design applied to stripe rust {Puooinia 

 striiformis) in winter wheat 



Disease severity 

 Cultivar (fraction of leaf area diseased) 



Heines VII 0.105 



1 to 1 mixture 0.013 



Panter 0.000 



a The Netherlands, 1958, unreplicated plots, plot size 1 ha, 

 natural infection, weather not very favorable to rust. 



A third trick has been proposed by several authors (see Johnson, 

 1958). The host area is divided into sub-areas, every sub-area being 

 protected by another monogene . In this way, a mosaic of resistance genes 

 is created. In the North-West-European wheat belt, a comparable situa- 

 tion was unconsciously created by national regulations; sometimes the 

 trick was effective (Zadoks, 1961). 



In this rather gloomy perspective for differential resistance there 

 is one spark of light. The epidemiological system under consideration 

 is a host-host-pathogen system (van der Plank, 1968), or more precisely 

 a Pinus-Bibes-Cronartium system. Differential selection in the pine 

 phase will possibly be counter-balanced by stabilizing selection in the 

 ribes phase. 



POLYGENIC RESISTANCE 



When man wants to speed up the work that nature does too slowly, 

 the conscious exploitation of intraspecific variations in resistance 

 seems the most promising way. The system uses well adapted parents and 

 does not introduce unwanted germ plasm. However, full resistance is 

 difficult to obtain within a few generations(Bingham, 1966) . Therefore 

 it is necessary to establish accurately what level of partial resistance 

 is acceptable. The purpose of this type of breeding is not to eliminate 

 the rust, but to live with it. 



The ecosystem of blister rust on western white pine is not only a 

 host-host-pathogen system but also an environment-host-pathogen-system. 

 The environment -pathogen relation has been studied extensively. The 

 results indicate the existence of danger areas, where the rust readily 

 infects the pine, and of fringe areas, where the frequency of infection 

 is reduced because of specific ecological conditions (Paterson and 

 Jewell, 1968; Van Arsdel, these proceedings. 



The environment-host-pathogen system has been little studied. For 

 example it is known that in colder areas the latent period of the rust 

 is prolonged, but this is little more than an environment-pathogen rela- 

 tion. The studies proposed here are investigations on the interactions 

 between environment, host genome and rust genome. Results of this type 



