Plant diseases and pests are a major constraint to agricultural production despite the various measures used to control them. Chemical control, although often e~~ective, may pose environmental hazards and is relatively expensive, especially in developing countries where it may be completely uneconomic. Control through genetically mediated resistance to diseases and pests, is both cheap and environmentally sa~e and at present most diseases and pests o~ staple ~ood crops are controlled through some form of resistance. One of the basic problems in the use of resistance is its ~re quent lack of durability; very often a type of resistance is used that 'breaks down' after a certain period. The temporary nature of this resistance, due to the development of new strains of pest or pathogen able to overcome it, has seriously hindered the improvement o~ the yield potential of many crops as a continuing effort is needed to replace old cultivars who resistance has failed, with new ones. Following Vanderplank's now classical publications (1963, 1968) which differentiated horizontal and vertical resistance, studies on several host-parasite systems have shown that di~ferent types of resistance can be distinguished genetically and epidemiologically, and on the ability o~ the pests or pathogens to adapt to them. A knowledge of how resistance operates at the population level has also opened up possibilities of 'managing' relatively simple resistance types in such a way that a stable host-pathogen system can be pro duced with a minimum of crop loss.

Inhalt
I Durable Plant Resistance and the Genetics of Host/Pathogen Populations.- Durable Resistance and Agricultural Development.- Genetic Background of Durable Resistance.- Present Knowledge and Theories Concerning Durable Resistance.- Durable Resistance in Crops: Should the Concept of Physiological Races Die?.- Theoretical Resistance Models.- Models Explaining the Specificity and Durability of Host Resistance Derived from the Observations on the Barley Puccinia hordei System.- Pathogen Fitness in Cereal Mildews.- Problems in Estimating Parasitic Fitness.- Response of a Powdery Mildew Population to a Barley Variety Mixture.- Changing Gene Frequencies in Populations.- Durable Resistance and Disease Complexes.- II Parameters and Targets of Durable Resistance.- Durable Resistance and Host-Pathogen-Environment Interaction.- Epidemiological Parameters of Plant Resistance.- Durable Resistance in Crops: Biometric Analysis of Resistance.- Expression of Incomplete Resistance to Pathogens.- Resistance of Crops to Nematodes.- Durable Resistance of Barley Cultivars to the Nematode Heterodera avenae.- Resistance to Insects Promotes the Stability of Integrated Pest Control.- Insect Resistance in Cotton in Sudan.- Breeding Cotton Cultivars Resistant to Whitefly.- III Breeding for Durable Resistance and its Practical Application.- Pathosystem Management.- The Socio-Economic Implications of Breeding for Durable Resistance in Developing Countries.- Durable Resistance in Perennial Crops.- Breeding Elm, Pine and Cypress for Resistance to Diseases.- Research for Durable Resistance to Vascular Wilt Disease (Fusarium oxysporum f. sp. elaeidis) of Oil Palm (Elaeis guineensis).- Incomplete Resistance to Coffee Leaf Rust.- Resistance to Coffee Berry Disease in Ethiopia.- Durable Resistance inPotatoes for Developing Countries.- Genetic Recombination and Mode of Inheritance of Pathogenic Characters by Phytophthora infestans through Sexual Reproduction.- Durable Resistance in Self-Fertilizing Annuals.- International Testing as a Means to Implement Durable Resistance: The Experience of CIMMYT with Wheat Rusts.- Breeding for Disease Resistance in Wheat, the Zambian Experience.- Breeding for Disease Resistance in Wheat: the Brazilian Experience.- Experience of using Durable Resistance in the USA.- Durability of Barley Powdery Mildew Resistance Genes in Denmark 19631980.- Disease Resistance in Rice.- Evaluation of Faba Bean Cultivars for Slow Rusting Resistance.- Interactions between Pearl Millet Varieties and Salerospora graminicola isolates.- Participants.- Lecturers.