In modern wheat cultivars, resistance to wheat leaf rust, Puccinia triticina , is either based on hypersensitivity resistance or on partial resistance. Hypersensitivity resistance in wheat is monogenic, often complete and posthaustorial: it is induced after the formation of a haustorium by the pathogen in the plant cell. The localised plant cell death that follows prevents the uptake of nutrients by the pathogen. Although this type of resistance is widely used in wheat breeding, it is in general not durable as the pathogen overcomes the resistance with new variants. Partial resistance is mono- or polygenic, has a quantitative expression with a susceptible infection type and is assumed to be durable. A small proportion of the infection units in partially resistant wheat is aborted prehaustorially, before the formation of haustoria by the fungus, whereas the other infection units continue to grow and form pustules. Partial resistance is not associated with necrosis.Diploid wheat is a close relative of tetra- and hexaploid cultivated wheats and can be used as a donor species in wheat breeding. Certain diploid wheat accessions show complete resistance to wheat leaf rust based on a high level of prehaustorial resistance as compared to hexaploid wheat. The remaining infection units are aborted in association with hypersensitivity. Transferred to cultivated wheat, the high level of prehaustorial resistance from diploid wheat might provide an interesting and possibly durable alternative for posthaustorial resistance in wheat breeding. The occurrence of resistance in diploid wheat, the level of prehaustorial resistance and the inheritance of prehaustorial resistance are the subject of this thesis.Diploid wheat comprises three species, T.monococcum , T. boeoticum and T. urartu , that can be distinguished based on morphological and molecular characteristics. The frequency of resistance in diploid wheat was assessed by determining the reaction of about 200 accessions per species to the wheat leaf rust isolate Felix. Resistance and susceptibility to wheat leaf rust was almost perfectly associated with species identity. T. monococcum was almost completely resistant (98%), whereas T. boeoticum and T. urartu were completely susceptible.The percentage of early aborted infection units without and with necrosis in resistant T. monococcum accessions was determined to assess the level of pre- and posthaustorial resistance, respectively. Resistant accessions with a truly high level of prehaustorial resistance were scarce. Two percent of the accessions had a level of early abortion without necrosis of 50% or more in the seedling stage. The percentage of early abortion without necrosis in resistant seedlings ranged from 0% to 80% and all remaining infection units were aborted with necrosis.The association between species identity and leaf rust reaction in diploid wheat and the combination of pre- and posthaustorial resistance in T. monococcum resemble nonhost resistance of cereals to inappropriate rust species, suggesting that T.monococcum is a nonhost for Puccinia triticina .To elucidate the inheritance of prehaustorial resistance, two mapping populations were studied for their reaction to wheat leaf rust. One population consisted of 72 recombinant inbred lines (RILs, F 6 ) of the resistant T. monococcum DV92 and the susceptible T. boeoticum G3116. The other mapping population was an F 2 of 118 plants of the susceptible T. boeoticum Tb1486 and the resistant T. monococcum Einkorn. Both resistant parents had a relatively high level of prehaustorial resistance to wheat leaf rust. For the RIL population an F 2 -based RFLP linkage map already existed to which AFLPs were added and for the F 2 population a maternal and paternal AFLP map were produced.Two quantitative trait loci (QTLs) were identified: one on chromosome 5 of all three maps and one on chromosome 4 of the maternal F 2 map. The QTLs had a pleiotropic effect, governing prehaustorial as well as posthaustorial resistance, and were effective in the first and the fifth leaf stage. In addition, two putative QTLs, for pre- and posthaustorial resistance, effective in the first leaf stage only, were identified: one on chromosome 3 of the RIL map and one on linkage group 5 (unassigned) of the paternal F 2 map.It is remarkable that the two different resistance mechanisms in diploid wheat are controlled by the same QTLs. In barley and cultivated wheat, genes for prehaustorial resistance and hypersensitivity resistance represent two different classes that map to distinct locations on the respective genomes. The QTLs from diploid wheat, identified in this study, might represent a third class of genes for wheat leaf rust resistance.The pleiotropic effect of the QTLs for prehaustorial resistance from diploid wheat may prevent the introgression into cultivated wheat of prehaustorial resistance alone. Still, the resistance from diploid wheat accessions with a high level of prehaustorial resistance, such as Einkorn and DV92, could prove useful in wheat breeding, conferring a possibly durable resistance similar to nonhost resistance.Anker CC, Niks RE (2000)Genetics of prehaustorial resistance to wheat leaf rust in diploid wheat. Acta Phytopath Entomol Hungarica 35: 23-30Anker CC, Niks RE (2001)Prehaustorial resistance to wheat leaf rust in Triticum monococcum (s.s.) Euphytica 117: 209-215Anker CC, Buntjer JP, Niks RE (in press)Morphological and molecular characterisation confirm that Triticum monococcum s.s. is resistant to wheat leaf rust. Theor Appl Genet