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ALS resistant prickly sowthistle found in the UK

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ADAS weed researchers, Dr Sarah Cook and Dr Laura Davies, have identified an ALS-resistant population of prickly sow thistle (Sonchus asper) for the first time in the UK. The population from the west midlands, originated in a field of spring wheat that has an extensive history of ALS-inhibitor herbicide use.

 

Figure 1: Prickly sow thistle (Sonchus asper) (photo from www.missouriplants.com)

ALS-resistant prickly sow thistles have already been identified in Canada, France, Norway, and the USA, with resistance to a range of sulfonylurea actives and imazamox.

Glasshouse tests showed the UK population is highly resistant at 30g/ha metsulfuron-methly (4 times recommended field rate) (Figure 2 and 3), as well as resistant to field rates of thifensulfuron-methly and imazamox.

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Figure 2: Dose-response of (a) sensitive and (b) resistant prickly sow thistle populations treated with eight doses of metsulfuron-methyl
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Figure 3: Dose-response curve for survival of suspected resistant (circle) and sensitive (triangle) prickly sow thistle populations treated with eight doses of metsulfuron-methyl

Control

Fortunately, at the moment the resistant prickly sow thistle can still be controlled using other herbicides (e.g. mecorprop-P). However, as with all weeds, an integrated weed management approach should be used to increase control and reduce the risk of resistance. Effective non-chemical methods of control for prickly sow thistle include soil cultivation (ploughing – high; shallow cultivation ~5cm depth – medium) and using stale seedbeds.

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