Breeding wheat for resistance to biotic stresses
Although common wheat and durum may be attacked by a large number of diseases and pests, less than 20 diseases and about five insect or mite pests are of major significance. Some of these have a global distribution and occur in most wheat-growing areas, whereas others are restricted to certain geographic regions or climatic zones. A small group of diseases and pests, such as Karnal bunt and Russian wheat aphid, are a major threat to countries and regions in which they are absent. Although there is genetic variation in response to most diseases and pests, resources available to national programs limit the number that can be included as breeding objectives. Ideal sources of resistance are those present in closely related, commercial genotypes, but care must be exercised to avoid genetic uniformity. Any effort to transfer resistance from related species and genera should be considered long term. The chance of successful exploitation of resistance based on alien genetic material declines with reducing genetic relatedness between recipient and donor species. Breeders must maintain an awareness of potential problems associated with very high levels of resistance controlled by single genes. Lower levels of resistance with established durability or resistance based on a number of genes may be preferred. In many situations, resistance with moderate to low effectiveness will contribute significantly to crop protection. Unusually susceptible genotypes should be avoided irrespective of perceived risk based on local surveys. Molecular and other markers for genes of interest are having an increasing role in the selection process. Although genetic engineering in wheat is in its infancy, significant contributions to disease resistance, starting with virus resistances, can be expected.