Chapter 8 - Diversity in resistance to biotic stresses

Barley, the fourth largest cereal crop globally suffers naturally from damage caused by a large number of organisms. The defence of a plant to biotic stress is a very important key for germplasm evaluation. Regions where the host and the pathogen are indigenous and have co-evolved are birthplaces for the development of different defence mechanisms and, consequently, of a high diversity. This chapter reviews the situation concerning diversity in the genus Hordeum with respect to resistance or tolerance to important diseases or pests. In particular, emphasis will be placed on descriptions of the taxonomic, the geographic and, where known, the genomic level of diversity. Resistance breeding is often a cumbersome task demanding sharp selection tools to be efficient. With the arrival of new marker techniques, however, selection methodology has taken a great leap forward and in many instances revolutionized the breeding work. The present state of the art regarding plant breeding to control the agent is also discussed as well as the current situation of use or future prospects for breeding resistant varieties. The chapter presents the insight into intricate host-parasite relationships.