ABSTRACT
Many of the major crops on which humans depend for food are constituted primarily of cultivars or landraces selected long before "modern" agricultural science began. Scientific breeding of plants for disease resistance probably did not begin until after the disastrous potato late blight epidemic in Ireland during 1845 and 1846. An estimated million Irish died in the resulting famine. The ancient Romans carefully selected grain for future plantings. Traditional agriculture, especially in the hot, humid tropics, is often highly diverse with regard to the number of species grown and the planting patterns and architecture of the agroecosystems used. Resistance to almost every pathogen for which pathologists and breeders have searched has been identified in South American potato germplasm. Erosion of genetic resistance to pathogens and pests, due to a few new varieties of crop plants replacing landraces, is a major concern today in the international agriculture community.
