46 



J. C. ZADOKS 



For the purpose of comparison between different tests the most 

 susceptible plant should be used as a susceptible control. For each 

 character the disease rating of the test plant - DIS(T) - is expressed 

 as a fraction of the disease rating of the susceptible control - DIS(S). 

 Relative resistance RES is found by deducting this ratio from 1, i.e., 

 by the formula 



_ DIS(T) 

 *" " i " DIS(S) * 



The data from measured components of resistance all can be included in a 

 combined index of relative resistance (as in Table 1) , which is again a 

 value between zero and one. 



Table 1. The combination of components of resistance into one 

 index, the "relative resistance", in potato late blight 

 (Phytophthora infestans) . This index places the cultivars in 

 the same ranking order as the official resistance rating based 

 on numerous field observations (after van der Zaag, 1956, 

 table 13) 



Components of resistance 

 DIS(T) with susceptibility 



=1 Relative Official 



Cultivar 



In 



fection lesion 

 sporulation 





Product 

 P 



resistance 

 RES = 1 - P 



resistance 

 rating 





ratio 



growth 



rate 





Eersteling 



DIS(S) 



1. 



1. 



1. 



1. 





.000 



.3 



Eigenheimer 



.4 



.6 



1. 



.24 





.760 



.5 



Voran 



.2 



.6 



.2 



.024 





.976 



.7 



Noordeling 



2 



.5 



.15 



.015 





.985 



.75 



RUST NURSERIES 



The components of resistance can be studied in the laboratory, but 

 since laboratory tests are labor consuming they are usually bypassed in 

 favor of field tests. A typical field test design is that of the rust 

 "race nursery" (Fig. 2) . Wheat cultivars are sown in clumps or short 

 drills alongside a row of a highly susceptible cultivar. The latter, 

 called a "spreader", is inoculated with one isolate. A localized but 

 severe epidemic builds up and provides a gradually increasing amount of 

 inoculum which spreads over the test cultivars. The resulting disease is 

 assessed and a parameter of resistance calculated. 



The epidemic as observed on a test cultivar is composite in origin. 

 Part of the epidemic is due to the continuous influx of inoculum from the 

 spreader and part due to inoculum produced by the test cultivar itself. 

 Usually the spreader contributes most of the inoculum so that the epidemic 

 observed on the test cultivar is a mere reflection of the epidemic on the 

 spreader. In this case, the rust nursery test is little more than a "con- 

 tinuous monocyclic" test. This type of test underestimates the power of 

 partial resistance. 



