DISEASE RESISTANCE IN ANNUAL CROPS 49 



GENETICS OF RESISTANCE 



RESISTANCE SPECTRA 



Partial resistance is the result of the interaction between one test 

 plant and one isolate of the pathogen. The test plant can be a repre- 

 sentative of that population of nearly identical plants which is called 

 a "cultivar". This cultivar can be a clonal line descending from one 

 heterozygous hybrid (in potatoes) or a family of plants obtained by 

 inbreeding from a single homozygous parent (in wheat) . In a similar way, 

 the pathogenic isolate can be representative of that group of isolates 

 that is called a "physiologic race" because they have a combination of 

 pathogenic characteristics not occurring in other isolates. In dealing 

 with annual crops it is more precise to speak in terms of race-cultivar 

 interaction than of isolate-test plant interaction. 



A graphical representation of the "one race - one cultivar" inter- 

 action is given in Fig. 5. Resistance is depicted as a column with a 

 height 0<RES<1. The "one race - many cultivars" interaction is shown in 

 Fig. 6 as a colonnade. The resulting picture is called an "infection 

 spectrum". The picture of a "one cultivar - many races" interaction, 

 shown in Fig. 7, may be called a "resistance spectrum". A cultivar 

 showing high resistance to one and low resistance to another race 

 differentiates between these two races and can be used as a "differential 



UNIFORM AND DIFFERENTIAL RESISTANCE 



In some cases the resistance of the cultivar apparently is about 

 equal to all races tested. Van der Plank calls this phenomenon "uniform 

 resistance" (1969) , a better term than his earlier "horizontal resistance' 

 (1963; 1968) . Uniform resistance may reach different levels (Fig. 8) . 

 In striking contrast with uniform resistance is "differential resistance" 

 formerly called "vertical resistance" (van der Plank, 1968; 1969). The 

 resistance spectrum can show all values for RES from to 1 . The best 

 differentials for race identification are those which give either or 1 . 

 Examples can be found in potato late blight (P. infestans) and flax rust 

 {M. lini) . 



Typical uniform and typical differential resistance can occur 

 together as van der Plank (1963) showed for the potato cultivars Kennebec 

 and Maritta (Fig. 9) tested with p. infestans. This situation is called 

 "two-dimensional resistance". In field tests with stripe rust (P. 

 striiformis) of wheat, more complicated results were obtained which 

 emphasize the hypothetical nature of the concept of horizontal resistance, 

 Differential resistance can be recognized in cv. "Heines VII" but uniform 

 resistance in its typical form is absent from both cv. Heines VII and 

 cv. "Probus" (Fig. 10). 



