Final Report Summary - BARLEYNONHOST (GENETICS BASIS OF THE NONHOST RESISTANCE TO POWDERY MILDEWS AND RUSTS IN BARLEY) Full nonhost resistance can be defined as immunity, displayed by an entire plant species against all genotypes of a plant pathogen. The genetic basis of (non)host-status of plants is hard to study, since identification of the responsible genes would require interspecific crosses that suffer from sterility and abnormal segregation. There are some plant/potential pathogen combinations where only 10% or less of the accessions are at most moderately susceptible. These may be regarded as marginal host or near-nonhost, and can provide insights into the genes that determine whether a plant species is a host or a nonhost to a would-be pathogen, and to investigate the genetics of the host-status to specialized phytopathogens. Barley (Hordeum vulgare L.) is an example of a near-nonhost to several rusts (Puccinia) and powdery mildews (Blumeria graminis) of cereals and grasses. The research group has developed experimental lines with extreme and unusual susceptibility to different heterologous rusts and powdery mildews. These lines and fully resistant accessions are used to enhance the knowledge on the genetic basis of the resistance at the species level (nonhost resistance) to rusts and powdery mildews in barley. This research is unique in its approach and will add new scientific insights, complementary to the nonhost research elsewhere. Understanding which genes are involved in natural variation between plant species in (non) host status to pathogens may lead to improved strategies to apply durable forms of disease resistance in breeding programs When the fellow started, there were several mapped QTLs for resistance to heterologous rusts. Most QTLs were mapped with a precision of about 5 to 20 cM. The fellow therefore mapped and fine-mapped several of these QTLs in barley. He developed a consensus linkage map in barley to associate QTLs for resistance to rusts and powdery mildews. This map was a great help in fine-mapping of Rnhq, a QTL for nonhost resistance to rust fungi, three QTLs effective for resistance to P. persistens and two effective to P. hordei bulbosi and of genes located near Rphq11 that were effective to several heterologous rust fungi.