REFLECTIONS ON DISEASE RESISTANCE IN ANNUAL CROPS 



J. C. Zadoks 



Laboratory of Phytopathology 3 Agricultural University 

 Wageningen, The Netherlands 



.ABSTRACT 



Assessment of partial resistance uses typological and quanti- 

 tative methods. Epidemiological theory suggests several 

 parameters that can be counted or measured, the so-called 

 "components of resistance". Laboratory tests with single 

 plants are "monocyclic" tests; race nurseries are "continuous 

 monocyclic" tests. For "polycyclic" tests, involving 

 populations of plants, specially designed field experiments 

 are needed. The "one cultivar - many races" test leads to 

 a "resistance spectrum" in which uniform resistance, differ- 

 ential resistance or both together can be discerned. Differ- 

 ential resistance is mono- or oligogenic; uniform resistance 

 polygenic. Environment can influence gene expression. 

 Natural selection for resistance uses polygenes and leads to 

 uniform resistance, an example to be followed by breeders. 

 Breeding for differential resistance is easy but differential 

 resistance tends to be self-destructive. To control white 

 pine blister rust, monogenes are not advisable but risks may be 

 reduced by the "composite design" and the "mosaic design". 

 Breeding for polygenic partial resistance is nature's own 

 method; it is the safest way to improvement; significant 

 interactions between environment and resistance (gene 

 expression) should be utilized when available. Many factors 

 can interact to reduce the relaxation time of an ecosystem 

 unbalanced by the introduction of a pathogen. 



INTRODUCTION 



Disease resistance is a desirable character which the breeder wants 

 to incorporate into his new synthetic plant types. All too often, disease 

 resistance is regarded as a qualitative character. The resistance is 

 either present and no disease symptom becomes visible, or it is absent 

 and plants become fully diseased after infection. 



This picture in black and white is misleading. A newly emerging 

 philosophy looks at disease resistance as a quantitative character, that 

 is a character which can assume all grey tones from pure white to raven 

 black. It is the art of the pathologist to measure the grey tone value 

 correctly. The possibility of accurately assessing intermediate forms of 

 resistance opens the way to new techniques of resistance breeding. 



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