Glyphosate-resistant Plantago lanceolata in the Western Cape.

In May 2003 a sample of Plantago lanceolata seed, which had been harvested from plants from an area that had a history of poor control with glyphosate, was received from an Agro-Chemical company for resistance evaluation. This population of P. lanceolata was found to be growing in a vineyard in the Breede Valley (about 100km north east of Cape Town). The seed was germinated in an incubator and the seedlings planted out in pots. Seedlings from two other P. lanceolata populations, one from an undisturbed area (Rondebosch Common, Cape Town) and the other from a cultivated land (Welgevallen Experimental Farm, Stellenbosch), were planted as controls. Six weeks after planting out, when the plants were in the 4 - 8 leaf stage, the three populations were subjected to a range of glyphosate dosage rates viz. 0.36, 0.72, 1.08, 1.44, 2.16, 2.88, and 3.60 kg a.e. glyphosate ha^-1. The Breede Valley population was also subjected to higher range of dosage rates viz. 4.32, 5.04, 5.76, 6.48, 7.20 kg a.e. glyphosate ha^-1. Four replicates per treatment were used.

No treatment gave acceptable control of the Breede Valley population (Table 1). Dose response between 4.32 and 7.20 kg glyphosate ha^-1 was flat and differences in the control levels within this dosage range were probably due to natural variability in the population. The level of control achieved of the Welgevallen and Rondebosch populations of P. lanceolata was acceptable. This trial was rated 56 days after treatment.

Table 1. Dose response of three Western Cape populations of Plantago lanceolata to glyphosate.

*nt = not tested

Work is in progress to map the extent of glyphosate-resistant populations of P. lanceolata in the Western Cape, and to identify cost effective control strategies.

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