EVOLUTION OF VERTICAL AND HORIZONTAL RESISTANCE AND ITS APPLICATION IN BREEDING RESISTANCE TO POTATO LATE BLIGHT

Abstract

The concepts of vertical and horizontal resistance in plants are based on the number of “major†genes involved in the defence mechanisms of the host. Vertical resistance is monogenic or only few genes activate the defence reactions, while horizontal resistance is polygenic. In potatoes these approaches were identified in early 1900’s in Solanum demissum. However, in the 1950’s resistant American and European clones were infected by local races of Phytophthora infestans (Mont) de Bary in the highlands of central México, alerting breeders and pathologists to “new†races of the pathogen previously unknown to science. In addition, characterization of such races with recent molecular tools creates a new paradigm for variation in the pathogen independent of the old gene-for-gene concept of host/pathogen resistance/virulence. Sexually derived progenies, great number of propagules, short life cycles, and high frequency of genetic changes, provide P. infestans with enough pathogenic plasticity to easily breakdown genetic resistance of the host. This leads to a continuous struggle to generate more and better potato clones resistant to the pathogen including the transgenic approach.