Herbicide-Resistant Weeds in Europe: Agricultural, Physiological and Biochemical Aspects

Abstract

In Europe, cultivated land is mainly given over to corn (11.4 Mhas), wheat and barley (40.75 Mhas) and orchards (40.5 Mhas). For a long time, the continuous use of conventional herbicides such as triazines, phenylureas, aryloxyphenoxypropanoates, cyclohexanodiones, sulfonylureas and imidazolinones has been made but without any rotation of crops and herbicides. Now, 35 broadleaf weeds and 17 grass weeds have been identified as being resistant to various herbicides. The first herbicide-resistant biotype was a triazineresistant Amaranthus retroflexus found in Austria in 1973. The proposition of the mutation of the target site as a resistance mechanism was first made in France in 1978, using chlorophyll fluorescence and Hill reaction measurements in a triazine-resistant Chenopodium album biotype. In 1987, the same researchers showed that the resistant C. album biotype had Ser264 to Gly mutation in the psbA gene. Triazine-resistant broadleaf weeds such as Amaranthus bouchoni, A. hybridus, A. retroflexus, C. album and Erigeron canadensis showed cross-resistance to some phenylureas, carbamates and uracils in Hungary. The use of bipyridyl herbicides in orchards has selected resistant biotypes of Epilobium ciliatum, E. canadensis, Erigeron bonariensis, Poa annua and Lolium perenne. However, various resistance mechanisms to paraquat have been proposed, according to the weed species. ALS inhibitors have been widely used in Europe, and their repeated use on crops leads to the rapid appearance of resistance. However, only three sulfonylureas-resistant biotypes of the Stellaria media, Alisma plantago-aquatica, and Papaver rhoeas species have been detected in Denmark, Portugal and Spain, respectively. In the case of grass weed, eight triazineresistant biotypes have been detected: five of these showed resistance due to the target site mutation and the other three showed natural glutathione-atrazine formation such as a nontoxic compound. Resistance to chlorotoluron and/or diclofop-methyl due to enhanced detoxification has been confirmed in Alopecurus myosuroides, Apera spica-venti, Bromus tectorum and Lolium rigidum. Some of these biotypes have presented cross- and multiple-resistance to other herbicides due to a system such as Cytochrome P450. Recently, two cases of resistance to acetyl-CoA carboxylase (ACCase) herbicides have been detected: a Lolium rigidum in France and an Avena fatua in the U.K. The tolerance of L. multiflorum to aryloxyphenoxypropanoates (APP) is due to the presence of a tolerant form of ACCase. With respect to other herbicides, different Echinochloa crus-galli populations resistant to propanil and quinclorac have appeared in rice fields in Greece and Spain, respectively. The resistance mechanism to propanil is due to a detoxification via hydrolysis catalyzed by aryl acylamidase. The tolerance to quinclorac is due to the non-stimulation of the ethylene biosynthesis in resistant Echinochloa spp. Combatting broadleaf resistance is not difficult in crops like corn, rice and orchards in general, where crop rotation and/or a pre- and post-emergence mixture of herbicides can be used. Cross- and multiple-resistant grass weeds are more difficult to combat and require an urgent solution because more resistant new grass weeds are appearing every year.