DISEASE RESISTANCE IN ANNUAL CROPS 47 



RACE NURSERY 'continuous monocyclic' test 



1 2 3 k 5 6 7 8 9 10 11 12 13 U 15 16 17 18 1920 

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20 19 18 17 16 15 K 13 12 11 10 9 8 7 6 5 4 3 2 1 



O test hill 



© spreader hill ► 1 m 



• inoculated spreader hill 



Figure 2. Design of a rust race nursery for monocyclic testing 



of resistance to one race of Puceinia recondita in 20 wheat 



cultivars. Cultivar clump diameter is 25 cm, distance between 



centers of clumps 40 cm (Zadoks, 1965). 



POLYCYCLIC TESTS 



Underestimation of the power of partial resistance is safe, but 

 inefficient. The monocyclic test neglects the fact that an epidemic 

 often builds up by successive infection cycles. Small differences in 

 partial resistance as observed in a monocyclic test can have large 

 effects after a number of repeating infection cycles. The monocyclic 

 test is essentially an experiment with an individual plant. The poly- 

 cyclic test is an experiment with a population of plants; it adds another 

 dimension to disease resistance testing. 



Field tests specially designed to evaluate the cumulative effects 

 of partial resistance (Fig. 5) , resemble small fields inoculated at the 

 center. Amount and spread of the disease are assessed at regular 

 intervals and parameters of susceptibility or resistance calculated 

 accordingly. These tests reveal that some cultivars, highly suscep- 

 tible according to their infection type, are"slow rusters" (Fig. 4). 

 In slow rusters the epidemic builds up at a reduced rate and the 

 resulting damage and yield losses are reduced accordingly (Romig, 1964) . 



It is not easy to combine accurate resistance tests with accurate 

 yield tests, but many breeders have experienced that some cultivars yield 

 better than others even when they have exactly the same level of disease 

 (Clifford and Schafer, 1968). This phenomenon is called "tolerance" 

 (Simons, 1966). Tolerance may be defined as resistance to damage. The 

 genetics of tolerance are unknown and breeding for tolerance is only 

 beginning to be practiced (BrOnnimann, 1968). 



