Paraquat-resistant Conyza bonariensis in the Western Cape.

In January 2003 reports were received from the Breede Valley (about 100km north east of Cape Town) of glyphosate failing to control Conyza bonariensis at registered dosage rates. During the course of these investigations paraquat was evaluated as an alternative treatment to control the glyphosate-resistant weeds. It soon became clear that several of the control populations evaluated, which were susceptible to glyphosate, were resistant to paraquat. A paraquat dose response pot trial was carried out on one of these populations. Dosage rates applied were 0.6, 1.2, 1.8, 2.4, 3.0, 3.6, 4.2, and 4.8 kg a.i. paraquat ha-1. Seed was collected on the University of Stellenbosch’s experimental farm, Welgevallen which is situated about 40km from Cape Town. A population originating from seed collected at Goudini in the Breede Valley was used as a control. The trial was carried out on transplants in the rosette growth stage. Four replications of 4 plants per pot were used. From Table 1. it is clear that the Welgevallen population was resistant to paraquat. Virtually no dose response was observed in the Welgevallen population. The Goudini control population was well controlled by paraquat.

1. Dose response of two Western Cape populations of Conyza bonariensis to paraquat.

Provisional screening of 12 C. bonariensis populations from the Western Cape showed that 7 of them showed significant resistance to paraquat. Generally speaking, all populations evaluated were controlled by either paraquat or glyphosate. However, one Breede Valley population appeared to be resistant to both herbicides. This population is the subject of further investigation.

A.L.P. Cairns
Department of Agronomy University of Stellenbosch