52 



J. C. ZADOKS 



RESISTANCE 



1 2 3 4 12 1.3 1.4 23 24 3,412312,4 1.3.42.3.4 1/4 RACES 



RESISTANCE 



3 4 12 13 14 23 24 3.4 12.3 12413.423.4 1/4 RACES 



Figure 9. "Two-dimensional resistance" as demonstrated by the 

 interactions of potato cultivars Kennebec (A) , and Maritta (B) , 

 with Phytophthora infestans. Both cultivars carry the hyper- 

 sensitivity monogene Rl. This gene imparts complete, differen- 

 tial resistance to the races 0, 2, 3 and 4, and to the complex 

 races 2,3; 2,4; 3,4; and 2,34 of the pathogen. Kennebec also 

 displays a low, and Maritta a medium level of uniform resis- 

 tance (after van der Plank, 1963). 



