Assessment of glyphosate-resistant horseweed (Conyza canadensis L. Cronq.) and fleabane (Conyza albida Willd. ex Spreng) populations from perennial crops in Greece

The extended use of glyphosate resulted to its reduced efficacy against increasingly problematic weeds, such as Conyza spp. The objectives of this study were to determine the occurrence of glyphosate resistance in horseweed (C. canadensis) and fleabane (C. albida) populations in Greece, to evaluate the effect of weed growth stage on glyphosate efficacy under controlled environmental conditions and to study seed germination patterns of glyphosate-resistant (GR) and glyphosate-susceptible (GS) populations. Plants from 28 and 14 populations of horseweed and fleabane, respectively, sampled from five prefectures in Greece were sprayed with glyphosate at recommended rates. 68% of the tested populations of horseweed were potentially resistant or intermediate, while the relative percentage for fleabane was significantly lower (50%), probably because of the later introduction of this species. After initial screening, six populations from each species were selected and dose-response experiments were conducted. Glyphosate rates required to control some populations were 7 to 14 times greater than that for control of the reference susceptible populations. Sensitivity of GR horseweed and fleabane populations to glyphosate was strongly dependent on growth stage, with plants at the seedling stage being most sensitive to the herbicide. Moreover, when seeds of GR and GS populations from both species were subjected to different alternating temperature, germination occurring and seedling vigour did not differ between them with maximum germination at 10/20 and 15/25 oC. Consequently, various integrated management strategies should be urgently implemented in order to manage or slow the spread of glyphosate resistance in these species. Keywords: Glyphosate resistance; Horseweed; Fleabane; Growth stages.