DIVERSITY AND MANAGEMENT OF RUSSIAN-THISTLE (SALSOLA TRAGUS L.) IN THE DRYLAND CROPPING SYSTEMS OF THE INLAND PACIFIC NORTHWEST
Spring, John Forrest
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Russian-thistle (Salsola tragus L.) is one of the most troublesome weed species in the low- and intermediate-precipitation dryland wheat-fallow cropping zone of the inland Pacific Northwest (PNW). High levels of morphological diversity typify the species on global, continental and regional scales. Previous research in California found this variability to encompass a largely cryptic complex of five distinct species in populations of Salsola in that state. Russian-thistle also exhibits high levels of morphological diversity in the inland PNW, suggesting that similar levels of genetic differentiation and population structure may be present in the region. A double-digest RAD-seq approach was used to characterize the genetic diversity and population structure of Russian-thistle in a sample of 94 individual plants collected across the wheat-fallow production region of eastern Washington and northeastern Oregon. Only one species (Salsola tragus sensu lato) was found. Multi-dimensional scaling, kernel-PCA-and-optimization population clustering, and Moran’s eigenvector mapping approaches all indicate the presence of a single, unstructured population across the region. High levels of standing genetic diversity were indicated in this population expected multilocus heterozygosity of 0.35. Separate trials related to field weed management led to the first reported case of glyphosate resistance in Russian-thistle from Washington state. Dose response experiments conducted in greenhouse and field settings indicated 2.5 to 8-fold resistance to glyphosate in this accession. Variability in the magnitude of expressed glyphosate resistance appeared closely correlated with temperature conditions after application, with higher temperatures resulting in higher levels of expressed resistance. Resistance to glyphosate poses a substantial threat to successful weed control in chemical fallow in the PNW, which relies heavily on glyphosate. Field experiments were conducted to evaluate several pre-emergence herbicides for addition to glyphosate-only chemical fallow management programs in the wheat-fallow region. Initial results indicate that this approach has potential to improve efficacy of control and reduce selection pressure for glyphosate resistance in chemical fallow, but further experimentation is needed before robust field management recommendations can be developed.