15 March 2002

Poppies and chickweed resisting key herbicides

By Andrew Blake

POPPIES resistant to the widely used sulfonylurea group of herbicides have been found for the first time in the UK. New cases of sulfonylurea-resistant chickweed have also been found.

Tests on poppy samples from three farms in the south and east of England confirmed the sulfonylurea resistance, says IACR-Rothamsted weed specialist Stephen Moss.

The finding comes after resistance was first detected in chickweed on a Scottish farm last spring. As a result, agronomists experiencing poor weed control were encouraged to get seed samples tested.

Resistance was confirmed in all nine samples received, including two chickweed samples and three poppy samples from south and east England.

"Clearly it would be misleading to assume we have extensive resistance," says Dr Moss. "But there is certainly more out there. The big question is how much?

"I dont think it need be considered anything like as big a problem as resistance in grassweeds because we have more alternative chemical options. But its a warning. There is no reason why resistance in other broad-leaved weeds wont arise."

Preliminary tests show the resistant poppies can be controlled by standard doses of non-ALS herbicides, such as MCPA and pendimethalin (as in Stomp). Mecoprop and fluroxypyr can deal with resistant chickweed, he adds.

However, some of the chickweed resisters also showed some cross-resistance to florasulam (as in Boxer), which is also an ALS inhibitor.

Worldwide, 71 weed species have evolved resistance to sulfonylureas or one of the other four ALS herbicide groups. But cross-resistance patterns are complex, says Dr Moss. "Most problems tend to occur where growers are over-dependent on sulfonylureas."

Revised advice from the Weed Resistance Action Group to help growers avoid resistant broad-leaved weeds is being considered for next season. In the meantime, growers are advised to stick to basic WRAG anti-resistance tactics and:

&#8226 Maintain good field records to monitor and plan herbicide inputs.

&#8226 Investigate odd patches of unexplained poor broad-leaved weed control.

&#8226 Contact agronomist and consider resistance test.

&#8226 Avoid repeated use of herbicides with single mode of action, such as ALS inhibitors, without tank-mixing and/or sequencing with others which work in other ways.

&#8226 Ensure components in tank-mixes and sequences have high levels of activity against target weeds.

&#8226 Incorporate cultural controls where practical. &#42

Herbicide resistance

&#8226 First time in UK poppies.

&#8226 More cases in chickweed.

&#8226 Less concern than in grasses.

&#8226 Revised WRAG advice likely.

&#8226 First time in UK poppies.

&#8226 More cases in chickweed.

&#8226 Less concern than in grasses.

&#8226 Revised WRAG advice likely.