Chemical Control of Fleabane Resistant to 2,4-D

Abstract

Conyza spp. (fleabane) are among the main weeds found globally. They have an annual life cycle and herbaceous size, high seed production, are found in different agricultural environments, such as grain crops (Photo 1). Fleabane control can be hindered by one or more of the following factors: high propagule production, wind propagation of propagules, herbicide resistance. For example, a single fleabane plant can produce more than 200,000 seeds in a year. In Brazil, there are cases of Conyza sumatrensis with multiple resistance to the herbicides chlorimuron and glyphosate, simple resistance to paraquat, in addition to cases of simple or multiple resistance to these and other herbicides (photosystem II inhibitors and synthetic auxins). Also in Paraguay, a country that borders Brazil, there is a report of a biotype with triple resistance to the herbicides paraquat, chlorimuron and glyphosate. In addition to this scenario of difficulties in control and cases of resistance, only 2.7 fleabane plants m–2 can reduce soybean productivity by 50%. These cases of fleabane resistance to herbicides make their management difficult, due to the loss of control tools. One of the main problems in this regard is resistance to 2,4-D, due to the complex rapid necrosis resistance. Rapid necrosis is an uncommon response to 2,4-D herbicide and this response is related to environmental conditions during application (temperature and light). After application of 2,4-D, necrotic symptoms occur rapidly in the leaves, in some situations within less than an hour after application. The rapid accumulation of reactive oxygen species (ROS) after application of 2,4-D causes lipid peroxidation and the production of ROS is related to the occurrence of necrosis in C. sumatrensis. Rapid necrosis resembles a programmed cell death response. However, these symptoms do not lead to plant death, with subsequent plant recovery one to two weeks after application, in addition to disrupting sequential application with burndown herbicides. Considering this setback, alternatives to be used to control fleabane iinclude the use of other synthetic auxins, such as dicamba, triclopyr, halauxifen or fluroxypyr, or even other herbicides, such as atrazine and mesotrione, which are adopted in maize cultivation or in the off-season. These herbicides can be used in the first application in a mixture with glyphosate, in which the control is complemented by the sequential application of glufosinate. These herbicides mixed with glyphosate are believed to be effective in controlling 2,4-D-resistant fleabane in the first application of pre-sowing soybean desiccation. The objective of this study was to evaluate the efficacy of glyphosate in mixtures with dicamba, triclopyr, halauxifen/diclosulam, fluroxypyr/clethodim, 2,4-D or atrazine/mesotrione, with sequential application of glufosinate, in the control of 2,4-D resistant fleabane in soybean pre-sowing.