Glyphosate is the world’s most widely used herbicide, with many registrations in agricultural, urban and semi-natural environments. Glyphosate is an important tool especially for broad-spectrum weed control and inter-row vegetation management in perennial crops like olive and citrus groves, orchards and vineyards. Despite the frequent use in these crops, there are only a few confirmed cases of glyphosate resistant weed populations in Europe. The first reported European case dates back to 2004 and involved four populations of Conyza bonariensis found in southern Spanish olive groves. The glyphosate rates required to control resistant populations were 7 to 10 times higher than those needed to control the susceptible populations. In 2006 a resistant population of C. canadensis was reported in southern Spain while another one was claimed in the Czech Republic in 2007. The resistance in Czech Conyza is currently still under investigation. Recently glyphosate-resistant Lolium rigidum was found in French vineyards (2005 and 2007) and in Spanish citrus groves (2006), while resistant L. multiflorum was identified in Spanish olive groves (2006). New cases of resistant Lolium spp. are reported from Italian vineyards and olive groves. In all these cases Lolium plants were not controlled at recommended glyphosate field rates. Although glyphosate resistant weed populations in Europe are only a few, the sole reliance on glyphosate for weed control especially in perennial crops bears the risk of selecting more resistant populations. Resistance management should be based on principles of Good Agricultural Practices and Integrated Weed Management:
– use the right rate at the right time;
– apply glyphosate when the plants are more susceptible and the environmental conditions are favourable for its uptake;
– integrate herbicides with different mode of action (e.g. a selective or residual herbicide) and mechanical weed control in the weed control program.
The new EU legislation on plant protection products is likely to decrease the number of active ingredients and herbicidal mode of actions available for weed control and therefore introduces new challenges for managing weed resistance.