Potato growers whose crops are unexpectedly hit by cyst nematodes may need to consider changing control products.

But TAG’s Denis Buckley stresses that they should do so only after eliminating all obvious reasons for less than satisfactory results.

The need to switch, he is fairly convinced, is that repeated use of nematicides with the same mode of action means they are broken down by soil microbes faster than usual.

“It’s very uncommon, but I know of four cases, two in the West Midlands, one near Peterborough and one in Yorks, where I’m pretty sure it’s at work,” says Mr Buckley.

“That’s because when we switched from a carbamate to an organophosphate nematicide, PCN control improved.

“If you return to a farm in early July where you recommended a carbamate to deal with a low egg count and the crop is dying, the silence can be very long.”

But several factors other than degradation can lead to poor nematode control, he points out.

They include too long a gap between applying the nematicide and planting the crop.

Simple things, such as whether the correct rate was used, also need eliminating as the cause of control failure, he adds.

“I don’t think it makes much difference whether products are applied on the destoner or the planter.

But quite often soil can block the applicator outlets.

“And on very light soils you can get leaching.”

Accelerated degradation is more likely where the field has a close potato rotation or a history of growing carrots, onions and sugar beet – all crops where soil-acting products with similar modes of action to the carbamates are used, he believes.

“You need to look at the history of the field.

In some cases growers may be using nematicides as often as one year in three or even tighter.”

Bill Parker of ADAS stresses that there have been no confirmed cases of enhanced breakdown of such products in UK fields.

But Dutch work in the 1980s found that where aldicarb (as in Temik) was applied over a five-year period, growers ran into trouble and had to switch products to regain nematode control, he notes.

More rapid breakdown could, in theory, allow the Globodera pallida species of nematodes, which hatch later than G rostochiensis, to escape and build up more than usual, Dr Parker acknowledges.

“The problem might be that their extended hatching period was still going on when the product had run out of steam.

“Certainly if it became an issue in the field then product selection would become even more important.

And as a matter of principle it is always good practice to alternate products where soil pesticides are applied annually to different crops in the same field.”

Changing to Nemathorin from, say, Temik or Vydate, should incur no extra cost, says Syngenta’s Jon Ogborn.

“They are all very similar in price.”

However, neither Nemathorin or Mocap, another potential alternative to carbamates, are necessarily immune to developing enhanced degradation over time, Mr Buckley believes.

“So a simple switch might not be a long-term solution.

We might need to look to alternating.”

andrew.blake@rbi.co.uk