44 J. C. ZADOKS 



MEASUREMENT OF RESISTANCE 



PARTIAL RESISTANCE 



The pathologist studies his pathogen by making isolates. Ideally, 

 each isolate is the genetically uniform clonal offspring of one single 

 cell. Different isolates may or may not have identical genomes. 



A test plant is tested for resistance by inoculating it with an 

 isolate under standardized conditions. In this "one isolate - one plant 

 test" one of the three following reactions can be expected: 



a. The plant remains healthy, resistance is complete. 

 Numerical value of resistance RES = 1. 



b. The plant becomes as diseased as the most susceptible control. 

 Numerical value of resistance RES = 0. 



c. The plant shows an intermediate reaction. 

 Numerical value of resistance 0<RES<1. 



In the latter case the resistance is said to be "partial". 



TYPOLOGICAL AND QUANTITATIVE ASSESSMENT 



There are two methods of disease assessment: The typological and 

 the quantitative method. The typological method uses a descriptive key 

 to characterize two or more classes of reactions denoted by symbols or 

 figures (see Fuchs, this proceedings). The observed disease reaction is 

 placed in one of these classes. Classification is easy in flax rust 

 {Melampsora lini) ] where the choice is between diseased and healthy, but 

 difficult in cereal rusts, where five main classes arbitrarily subdivide 

 a continuous range from absence of symptoms to severe disease. 



The quantitative method chooses characteristics which can be 

 measured or counted, e.g.: 



a. Infection ratio--the number of resulting lesions divided by the 

 number of spores applied. 



inoculation until first production 



b. Latent period--the period from i 

 of spores in the resulting lesions. 



c. Sporulation rate--the number of spores produced per lesion, per 

 unit of time. 



d. Lesion growth--the increase of lesion size per unit of time. 



e. Infectious period--the period during which lesions sporulate. 



These and other characteristics are well defined elements of 

 epidemiologic theory as summarized in van der Plank's (1963, 1968) 

 "mathematical model of resistance": 



1 Authorities for Latin binomials are given in the proceedings 

 subject index. 



