Technical Abstract: We developed an evolutionary relevant model system, barley-Puccini rust fungi, to study the inheritance and specificity of plant factors that determine to what extent innate nonhost immunity can be suppressed. A mapping population was developed from a cross between an experimental barley line (Suspired) with exceptional susceptibility to several heterogonous (nonhost) rust fungi and regular, immune, cv. Veda. Seedlings were inoculated with five heterogonous and two homologous (host) species of rust fungi. Resistance segregated quantitatively for each of the rust fungi. In total, 18 chromosome regions were implicated. For each rust species, a different set of genes was effective. Of the 18 chromosomal regions, 11 were significantly effective to only one rust species and 7 were effective to more than one rust species, implying genetic linkage or pleiotrophy. One resistance (R) gene for hypersensitive resistance to Puccinia hordei-secalini was mapped, suggesting occasional contribution of R genes to nonhost resistance in barley. Quantitative trait loci (QLTs) with effects to multiple rust fungi did not tend to be particularly effective to rust species that were phylogenetically related, as determine from their internal transcribed spacer sequence. We suggest that the QLTs described here play a role as specific and quantitative recognition factors that are specifically negated by the rust to successfully suppress innate immunity.