Cross-Resistance in and Chemical Control of Auxinic Herbicide-Resistant Yellow Starthistle ( Centaurea solstitialis ) 1

An accession of auxinic herbicide-resistant yellow starthistle found near Dayton, WA, was tested to evaluate cross-resistance to growth regulator herbicides and susceptibility to herbicides with different modes of action. Picloram at 0.43 kg ae/ha removed susceptible (S) yellow starthistle plants from a field plot, and surviving resistant (R) plants were dug and moved to the greenhouse. Known S plants were transplanted from a pasture near the R population. The R biotype was reconfirmed as resistant to picloram in greenhouse tests, with resistance ratios of 5.6 and 3.8 for vegetative and reproductive biomass, respectively, and 10.2 for LD50 (lethal dose for 50% of the treated population) data. The R biotype was also cross-resistant to clopyralid and dicamba in all responses and to 2,4-D and triclopyr in vegetative biomass and LD50 data. In field trials, eight herbicides were applied alone and in various combinations with and without addition of picloram. The yellow starthistle population was apparently comprised of differing percentages of R and S plants in 1997 and 1998, as picloram alone controlled 65 to 76% of the yellow starthistle in 1997 and 96 to 97% in 1998. BAS 662 01H (dicamba + SAN 836) at 0.28 or 0.42 kg ae/ha, respectively, or dicamba at 0.56 kg ae/ha were the best alternative treatments in either trial in either year, but only in 1998 did control exceed 85%. Picloram and other auxinic herbicides should continue to be useful for control of mixed R and S yellow starthistle populations. However, effective herbicides with different mode(s) of action integrated with range improvement practices and biological control must be identified for long-term yellow starthistle management. Nomenclature: BAS 662 01H (formerly SAN 1269; contains 20% ae 2-(1-[([3, 5-difluorophenylamino] carbonyl)-hydrazono] ethyl)-3-pyridinecarboxylic acid) (formerly SAN 836 [proposed common name diflufenzopyr]) plus 50% ae dicamba; yellow starthistle, Centaurea solstitialis L. #3 CENSO. Additional index words: Growth regulator herbicides, herbicide resistance, picloram. Abbreviations: I50, dose causing 50% reduction in vegetative or reproductive weight; POST, postemergence; R, resistant; S, susceptible,.