Technical Abstract: Tumbleweed or Russian thistle (Salsola tragus L.) is an introduced invasive weed in N. America. It is widely distributed in the U.S. and is a target of biological control efforts. The obligate biotrophic rust fungus Uromyces salsolae Richardt is a potential biological control agent of tumbleweed, and host range tests were conducted in quarantine to determine whether the fungus is safe to release in N. America. Results of the tests showed some non-target effects, but the tests were conducted under super optimum conditions and reflected only the response of the germ plasm tested. To arrive at anticipated disease reactions of each species, disease reaction data were combined with a relationship matrix derived from DNA sequences and analyzed with mixed model equations to produce best linear unbiased predictors (BLUPs) for each species. These results showed that: only species in the non-native genus Salsola were susceptible to the fungus; the analysis could be used effectively to generate BLUPs for rare and difficult to grow species; the analysis can be used to generate logical lists of plants to be tested for susceptibility.