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.