Mixed model analysis combining disease ratings and DNA sequences to determine host range of Uromyces salsolae for biological control of Russian thistle

Russian thistle or tumbleweed (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 Reichardt is a candidate biological control agent of Russian thistle, and host range tests were conducted in quarantine to determine whether the fungus is safe to release in N. America. Limited non-target effects were observed, but the tests were conducted under optimum conditions for disease and may reflect only the response of the germ plasm tested. To arrive at anticipated disease reactions of each species as a whole, disease reaction data were combined with a relationship matrix derived from DNA sequences and analyzed with mixed model equations (MME) to produce Best Linear Unbiased Predictors (BLUPs) of susceptibility for each species. These results showed that the MME were more useful than least squares methods in delimiting the host range of U. salsolae. Only species in the genus Salsola were susceptible to the fungus, and U. salsolae should be safe to release for the control of S. tragus in N. America. The MME were also used effectively in this study to generate BLUPs for rare or difficult to grow species that were not actually inoculated and to generate or validate lists of non-target plants for host range determinations. Advantages of the MME are discussed in light of host range determinations.