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Relative Susceptibility of Two Common Groundsel (Senecio vulgaris L.) Biotypes to Six s-Triazines¹

Effects of several s-triazine herbicides were studied on common groundsel (Senecio vulgaris L.) previously reported to be resistant to two chloro-triazine herbicides and on the same weed species from a location not previously treated with triazines. Herbicides tested were: 2-chloro-4,6-bis(ethylamino)-s-triazine (simazine), 2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine (atrazine), 2-(sec-butylamino)-4-(ethylamino)-6-methoxy-s-triazine (GS-14254), 2,4-bis(isopropylamino)-6-methoxy-s-triazine (prometone), 2-(tert-butylamino)-4-(ethylamino)-6-(methylthio)s-triazine (terbutryn), and 2,4-bis(isopropylamino)-6-(methylthio)-s-triazine (prometryne). The sensitive plants became chlorotic and died when treated with a triazine herbicide. Resistant plants never exhibited these symptoms. Plants of the sensitive biotype were effectively controlled by 0.5 ppm of atrazine or simazine, 1 ppm of GS-14254 or prometone, or 4 ppm of prometryne. The resistant biotype failed to show any chlorosis or necrosis at the highest rates tested, 4 ppm of simazine and 30 ppm for atrazine, GS-14254, prometone, and prometryne. Both biotypes were resistant to terbutryn at 30 ppm. These studies showed that resistance in one biotype was not restricted to chloro-triazines but extended to methoxy- and methylthio-triazines as well. The use of nutrient solutions rather than soil supported the conclusion that resistance is apparently due to physiological differences and not to differences in exposure to the herbicide caused by variation in germination tune, rooting depth, or morphology.

Key Words: Herbicides • Atrazine • Simazine • Prometryne • Terbutryn • Prometone • Weed resistance

² Graduate Fellow and Professor, respectively, Dep. of Agronomic Crop Science, Oregon State University, Corvallis, Oregon 97331. Senior author now Extension Specialist, Dep. of Botany, University of California, Davis, CA 95616.

Received for publication October 13, 1972.