Chickweed resistance first in Scots crops

23 March 2001

Chickweed resistance first in Scots crops

THE first case of broad-leaved weed resistance to sulfonyl-urea herbicides in the UK has been confirmed in Scotland, sending a timely reminder to growers of the risks associated with repeat use of such chemistry, say experts.

They stress that the case does not spell the beginning of the end for the ALS-inhibiting herbicide group.

"We dont want to give the impression this type of resistance is going to appear everywhere, but it is a new case," says IACR Rothamsteds Stephen Moss. "It is important growers are aware of this and monitor their own situation."

Resistance was suspected when Harmony M (metsulfuron-methyl + thifensulfuron-methyl) failed to control chickweed in spring barley last season. Seeds collected from the surviving weeds were sent to Rothamsted where pot tests confirmed target site resistance to metsulfuron-methyl (see table).

Similar resistance in chickweed has been seen in Ireland, Denmark and Sweden. Poppies, corn marigold and blackgrass are also considered as weeds most likely to develop ALS resistance by Du Pont.

Dr Moss believes blackgrass control is where the greatest risk from ALS-resistance lies, especially if the use of isoproturon is banned or further restricted.

Enhanced-metabolism resistance to Lexus (flupyrsulfuron-methyl) has already been recorded in blackgrass, he adds.

"The greater our dependence on ALS-inhibitors the bigger the problem. For broad-leaved weeds there are other herbicides you could use."

Du Ponts Martyn Rogers adds that all ALS-inhibiting herbicides, such as Lexus, Debut (triflusulfuron-methyl), or Titus (rimsulfuron) must be used in mixtures or sequences.

"We dont recommend Lexus or Debut as a stand-alone product, and rimsulfuron in potatoes follows a pre-emergence residual product such as Sencorex (metribuzin). All our recommendations are developed with sequences and mixtures."

"Spot the difference," says IACR Rothamsteds Stephen Moss. Treatment showed the Scottish chickweed (left)was resistant to sulfonyl-urea herbicide.

Loss of control

Foliage wt (g) % control

Untreated 19.7 –

Susceptible 1.4 93%

Resistant 16.5 16%

Chickweed 3.5 weeks after recommended rate application of Ally (metsulfuron-methyl).

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