Glyphosate-Resistant Hairy Fleabane (Conyza bonariensis) Is Reported in Greece Ilias S. Travlos and Dimosthenis Chachalis* 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 21 ; 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. Key words: Dose response, glyphosate resistance, herbicide, survey. El uso excesivo del glifosato puede incrementar los riesgos de reducir la eficacia del herbicida en las malezas y la resistencia de las mismas, incluso en regiones sin cultivos resistentes al herbicida anteriormente mencionado. Esa es la realidad en Grecia, con reportes frecuentes de la disminucio ´n de la eficacia del glifosato contra malezas cada vez ma ´s problema ´ticas, tal como la Conyza spp. Los objetivos de este estudio fueron determinar la ocurrencia de la resistencia al glifosato en poblaciones de Conyza bonariensis (L.) Cronq. ERIBO, en el centro y el sur de Grecia, el efecto de la etapa de crecimiento de la maleza sobre la eficacia de glifosato bajo condiciones ambientales controladas y evaluar herbicidas alternativos en ensayos de campo para el control de Conyza bonariensis (L.) Cronq. ERIBO tanto resistente como susceptible al glifosato. Las plantas de 60 accesiones de Conyza bonariensis (L.) Cronq. ERIBO, muestreadas de 5 prefecturas en Grecia, fueron rociadas con glifosato a 0.36 kg ea ha 21 ; 15 se clasificaron como potencialmente resistentes. Despue ´s de la seleccio ´n inicial, se escogieron 15 accesiones potencialmente susceptibles o resistentes y se realizaron con ellas experimentos de respuesta a las dosis. Las dosis de glifosato requerido para controlar algunas accesiones fueron de 4 a 7 veces mayores que la necesaria para el control de la accesio ´n susceptible de referencia, AR4. La sensibilidad de una accesio ´n de Conyza bonariensis (L.) Cronq. ERIBO resistente al glifosato, resulto ´ ser fuertemente dependiente de la etapa de crecimiento, siendo las plantas en la etapa de espigamiento, ma ´s sensibles al herbicida. Un estudio de campo mostro ´ que el diquat, glufosinate, o glufosinate+oxyfluorfen controlaron la Conyza bonariensis (L.) Cronq. ERIBO resistente o susceptible al glifosato. Estos herbicidas, usados con diversas estrategias integradas de manejo de malezas, tienen un buen potencial para controlar o reducir el avance de la resistencia al glifosato en estas especies. Conyza spp. include annual or short-lived perennials of North American origin that have become cosmopolitan and invasive weeds of many crops and arable lands (Michael 1977; Prieur-Richard et al. 2000). There are three main species of Conyza in Greece: hairy fleabane (Conyza bonariensis L.), horseweed [Conyza canadensis (L.) Cronq.], and fleabane (Conyza albida Willd. ex Spreng). Hairy fleabane and horseweed are the most common of the three species, whereas C. albida was more recently introduced (Economou et al. 2003). Today, there are many reports from Greek farmers that Conyza spp. have become increasingly difficult to control with several herbicides, especially in no-tillage or minimum-tillage systems (Travlos et al. 2009). Conyza spp. are prolific seed producers, with a single plant capable of producing thousands of nondormant seeds (Weaver 2001), which can be widely dispersed by wind (Shields et al. 2006). Conyza spp. have become common and problematic weeds in agronomic crops (Weaver 2001) because they adapt to periodically plant-free, undisturbed soil and establish in the absence of tillage (Brown and Whitwell 1988; Buhler 1992). Therefore, the opportunistic nature of Conyza in undisturbed areas makes them well suited for becoming established in agricultural fields (alfalfa, orchards, vineyards, vegetables, etc.) and surrounding areas, especially in no-tillage or minimum-tillage crop production systems (Brown and Whitwell 1988; Bruce and Kells 1990). Taking into account that over the last two decades the percentage of crop hectares in conservation tillage has significantly increased (Young DOI: 10.1614/WT-D-09-00080.1 * Lecturer of Agronomy, Laboratory of Agronomy, Faculty of Crop Science, Agricultural University of Athens, 75, Iera Odos St., 11855 Athens, Greece; Researcher, Department of Weed Science, Benaki Phytopatholigical Institute, 8 St. Delta Str., 14561, Kifissia, Greece. Corresponding author’s E-mail: htravlos@yahoo.gr Weed Technology 2010 24:569–573 Travlos and Chachalis: Glyphosate-resistant hairy fleabane in Greece N 569