Overreliance on glyphosate can increase the risks of reduced efficacy of the herbicide on weeds and weed resistance, even in regions without glyphosate-resistant crops. That is the reality in Greece, with frequent reports of reduced efficacy of glyphosate against increasingly problematic weeds, such as Conyza spp. The objectives of this study were to determine the occurrence of glyphosate resistance in hairy fleabane populations in central and southern Greece and the effect of weed growth stage on glyphosate efficacy under controlled environmental conditions and to evaluate alternative herbicides in field trials for control of glyphosate-resistant and -susceptible hairy fleabane. Plants from 60 accessions of hairy fleabane, sampled from five prefectures in Greece, were sprayed with glyphosate at 0.36 kg ae ha⁻¹; 15 were classified as potentially resistant. After initial screening, 15 potentially susceptible or resistant accessions were selected and dose–response experiments were conducted. Glyphosate rates required to control some accessions were four to seven times greater than that for control of the reference susceptible accession, AR4. Sensitivity of a resistant hairy fleabane accession to glyphosate was strongly dependent on growth stage, with plants at the seedling stage being most sensitive to the herbicide. A field trial indicated that diquat, glufosinate, or glufosinate oxyfluorfen controlled glyphosate-resistant or -susceptible hairy fleabane. These herbicides, along with various integrated management strategies, have good potential to manage or slow the spread of glyphosate resistance in this species.

Nomenclature: Diquat; glufosinate; glyphosate; oxyfluorfen; hairy fleabane, Conyza bonariensis (L.) Cronq. ERIBO.