In genetics, the term horizontal resistance was first used by J.E. Vanderplank to describe many-gene resistance. This contrasts with the term vertical resistance which was used to describe single-gene resistance. Raoul A. Robinson  further refined the definition of horizontal resistance. Unlike vertical resistance and parasitic ability, horizontal resistance and horizontal parasitic ability are entirely independent of each other in genetic terms.
In the first round of breeding for horizontal resistance, plants are exposed to pathogens and selected for partial resistance. Plants unaffected by the pathogen have vertical resistance and are removed. Obviously plants with no resistance die, the remaining plants have partial resistance and their seed is stored and bred back up to sufficient volume for further testing. The hope is that in these remaining plants are multiple types of partially resistant genes, and by crossbreeding this pool back on itself multiple partial resistance genes will be encouraged and provide resistance to a larger variety of pathogens than vertical resistance.
Successive rounds of breeding for horizontal resistance proceed in a more traditional fashion, selecting plants for disease resistance as measured by yield. These plants are exposed to native regional pathogens, and given minimal assistance in fighting them.
- Vanderplank, J.E. (1963) Plant Diseases: Epidemics and Control. Academic Press, New York and London, 349pp.
- Robinson, Raoul A. (1976) Plant Pathosystems. Springer-Verlag, Berlin, Heidelberg, New York, 184pp.
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