DISEASE RESISTANCE IN ANNUAL CROPS 



45 



dt 



(x 



t-p 



t-i-p 



) (1 - 



V 



In this equation x is the proportion of susceptible tissue at time t, 

 p is the latent period, i the infectious period, and R c is the number of 

 daughter lesions per mother lesion per unit of time. The epidemiological 

 characteristics described could be called "components of resistance". 



MONOCYCLIC TESTS 



There are two types of resistance tests: "monocyclic" and "poly- 

 cyclic". Cycle stands for infection cycle, the sequence of processes 

 from one generation of spores through infection, latency, etc., to the 

 next generation of spores. A single infection cycle from inoculation to 

 resulting sporulation can be studied in detail under laboratory conditions 

 A test involving one infection cycle on one plant is called monocyclic. 



Many laboratory investigations into the components of resistance are 

 reviewed by Hooker (1967) . The study of the infection ratio led to the 

 finding of a stomatal exclusion mechanism against leaf rust {Puooinia 

 vecondita) of wheat (Romig and Caldwell, 1964). Differences in resistance 

 of wheat to penetration of stem rust {Puooinia gvaminis) have been found 

 (Brown and Shipton, 1964) . Differences in latent period are obvious in 

 potato late blight {Phytophthora infestans ; Lapwood, 1961a) and stripe 

 rust (yellow rust, Puooinia striiformis) of wheat (Zadoks, 1961). Differ- 

 ences in spore production have been found in P. infestans (Lapwood, 

 1961b; Van der Zaag, 1956) and in leaf rust of wheat (Zadoks, unpublished 

 data). Usually, several components of resistance acting simultaneously 

 determine overall resistance (Fig. 1). 



> 

 w 20 



o 



2 10 

 < 



2 ° 



MICHIGAN A HUJ 



o 



Q. 



" 10 



> 



_l 



— 



1 



PERSIAN j 1 



u. 



HI 



m 



5 n 



FRANC NORD 



z 5 







" HEINE S VII ^ 







LEDA 









Figure 1. Components of resistance 

 in five winter wheat cultivars inocu- 

 lated with Puooinia striiformis race 

 B7X in the field when the flag leaf 

 was just visible; a = latent period, 

 b = average infection ratio (after 

 Zadoks, 1961, original data recalcu- 

 lated for 50 stems per cultivar) . 



5 10 15 20 



DAYS AFTER INOCULATION 



