54 J. C. ZADOKS 



To recapititulate the terminology, uniform, differential and two- 

 dimensional resistance refer to the resistance spectrum as displayed 

 under the one cultivar - many races situation. The preferred and the 

 earlier, synonymous terminology is given below: 



UNIFORM DIFFERENTIAL Van der Flank, 196$ 



horizontal vertical van dev Flank, 1963, 1968 



race -non-specific race-specific Zadoks, 1961 



generalized specific various authors 



Partial resistance also refers to the one cultivar - one race situation, 

 All these terms describe phenomena without any inference as to their 

 genetical background; they refer to phenotypes but not to genotypes. 



MONOGENIC RESISTANCE 



Differential resistance is usually inherited as a single dominant 

 gene (see Loegering, these proceedings) . Such genes are often called 

 "major" genes because they exert a major influence on resistance. To 

 avoid any allusion to size the term "major gene" could better be replaced 

 by the word "monogene". Major genes do not necessairly condition complete 

 resistance (RES = 1); there may exist major genes which condition partial 

 resistance. For instance, the Belgian wheat cv. "Alba" has differential 

 resistance to most stripe rust races in the Netherlands, but usually 

 shows 5 to 10 percent leaf infection towards the end of the growing 

 season (Zadoks, 1961). 



The phenotypic expression of a monogene for resistance can be 

 suppressed in some stages of the ontogenetic development of the plant. 

 There are numerous examples of genes conditioning resistance in the 

 mature plant but not in the seedling (Zadoks, 1961) or vice versa 

 (Sztejnberg and Wahl, 1967). Phenotypic expression of a resistance 

 monogene often depends on environment, especially on temperature (e.g., 

 Strobel and Sharp, 1965). The genie environment of a major gene can 

 also have an influence on its phenotypic expression; the effect is 

 ascribed to "modifier genes" (Knott and Anderson, 1956b; Toxopeus , 1958). 



During the last 50 years, resistance breeders utilized monogenes 

 because of the ease of handling (dominance) and the predictability of 

 the results. Moreover, several monogenes can be brought together into 

 one individual, the resulting resistance being called digenic, trigenic 

 or oligogenic, respectively (van der Plank, 1968). Oligogenic resistance 

 (genotype) is, as far as we know, always differential resistance 

 (phenotype) . 



POLYGENIC RESISTANCE 



Uniform resistance is supposed to have a different genetic basis. 

 A great number of genes regulate the normal function of a plant. Many 

 of these genes exert some effect on resistance. With respect to the 

 criterium "resistance" these genes behave as "minor genes". Here, minor 

 has the connotation of small in effect. When a minor gene can be identi- 

 fied by isolating its phenotypic effect from background noise it would 

 become a major gene for partial resistance. To avoid the terminological 

 problem van der Plank's (1968) term "polygene" is preferred. Polygenes 

 exist even in the absence of the pathogen, apparently because their 



