Multiple resistance in Conyza bonariensis puts pressure on management

In the agricultural heart of Rio Grande do Sul, an old enemy reinvents itself. conyza bonariensis, the buva, already dribbled the glyphosate. Now it also surpasses chlorimuron-ethyl (chlorimuron ethyl). The discovery came after analyses with biotypes collected in crops of soybean in the northwest of the state. The data indicate combined genetic and metabolic resistance, a warning to technicians and producers.

The study reveals that the biotypes tolerate up to 49 times the recommended dose of glyphosate. Against chlorimuron, the resistance factor exceeds 4900 in some cases. These numbers indicate a loss of practical efficiency in the field.

Genetic sequencing revealed three mutations: Pro106Thr in the EPSPS gene, linked to glyphosate; Pro197Arg and Trp574Leu in the ALS gene, associated with resistance to chlorimuron. In addition, tests with the malathion inhibitor demonstrated that accelerated metabolism, via cytochrome P450, participates in the breakdown of the herbicide.

In the trial, five biotypes were evaluated. Four were resistant to chlorimuron. Three were also resistant to glyphosate. Biotype II, with the Pro197Arg mutation, also showed cross-resistance to four other ALS inhibitors, such as imazethapyr and flucarbazone.

The proliferation of the species is impressive. As the scientists involved in the project pointed out, a single plant can release 800 seeds into the wind. Currently, infested areas cover more than 7 million hectares in Brazil. This represents about 30% of the area planted with soybeans, in addition to increasing the cost of control by about US$ 400 million per harvest.

Traditional herbicide rotation strategies lose strength in the face of multiple resistance. According to researchers, management needs to incorporate rapid molecular tests and the adoption of plant cover crops. Knowing the type of mutation present can guide more effective choices.

The study rules out an increase in the number of copies of the ALS and EPSPS genes as the cause of resistance. However, it points to increased gene expression in some biotypes. The combination of mechanisms – genetic and metabolic – raises a warning sign about the increasing complexity of chemical control.

More information at doi.org/10.1016/j.pestbp.2025.106501