USING GENETIC DIVERSITY FOR DISEASE RESISTANCE
C AKEM, S CECCARELLI, H TOUBIA-RAHME, W ERSKINE, S GRANDO and S AHMED
Germplasm Program, International Center for Agricultural Research in the Dry Areas (ICARDA), P0 Box 5466, Aleppo, Syria
More than 60% of global agriculture is practiced by subsistence farmers, most of whom live in developing countries and produce up to one quarter of the world's food. In contrast to modern agriculture, which depends on monoculture and the use of pesticides for disease control, subsistence farmers use practices such as mixed and intercropping and landraces or varietal diversity, which to a great extent have an impact on disease development. There has, however, been an increasing tendency towards uniform crop populations where resistance is controlled by a few major genes, but as uniform varieties are grown over wider areas, their vulnerability to disease epidemics is also increasing. The Southern corn blight outbreak in the USA maize crop in 1970 drew attention to this danger and highlighted the phenomenon of genetic vulnerability. Since then, continuous efforts have been directed at broadening the genetic base of crops by a search for sources of durable disease resistance, which remains the most practical and environmentally sound means for control of most major diseases.
The responsibility of plant breeders and pathologists in managing variation to combat genetic vulnerability is now of great importance. Three main germplasm resource bases available to do this are commercial varieties; landraces; and a wide range of wild ancestral species. This paper will review the exploitation of these diverse genetic resource bases for disease resistance in some selected crops. The presentation will draw heavily from research carried out at the ICARDA in Syria, and will also include case studies of research carried out elsewhere.