A new in-furrow approval for application of Syngenta‘s potato insecticide Actara (thiamethoxam) could help seed growers protect against early virus infections more effectively.

Foliar insecticide applications are successful in significantly reducing persistent viruses, such as potato leaf roll, but non-persistent viruses are proving more difficult to control, the firm’s Michael Tait says.

Over recent seasons up to 25% of seed crops entered for certification in England have been rejected for virus infection, with 90% of the cases testing positive being non-persistent viruses.

Seed potato plants can be rapidly infected with non-persistent viruses, such as potato virus A and potato virus Y, by aphid species, including cereal aphids, migrating through the crop, Mr Tait warns.

Plants are particularly vulnerable to virus infection during early growth stages. However, targeting very early foliar applications where there can be less than 20% ground cover is difficult and potentially wasteful.

An in-furrow treatment of Actara, where the insecticide is applied into the furrow at planting, is rapidly taken up by the establishing plant and moves into new growth as plants emerge, he explains.

Aphids feeding on the treated plants are quickly killed, preventing further damage and limiting the spread of virus infections, he adds.

Treatment should give around five weeks post-emergence protection from aphids, helping the plants through to a growth stage where foliar sprays are more effective.

The technique has been used to good effect by Dutch seed potato growers, where trials have shown good levels of aphid control 40 days after emergence, when aphid populations on untreated plants were multiplying rapidly.

Trials organised by Scottish Agronomy in 2009 also proved successful, with over 40% less infection in plots treated with Actara at planting and supported by a foliar treatment six weeks after emergence.

The trial, planted with PVA infector plants to create an artificially high risk situation, experienced a surge of virus transmitting grain aphids four weeks after the crop reached full emergence – before most growers would start a foliar programme.

Timing the start of a foliar programme is still important, even with Actara treatment, however, Mr Tait stresses. In the trial, delaying the foliar programme until eight weeks post-emergence, left levels of virus unacceptably high.

“The results reinforced the need for foliar treatments, but the in-furrow treatment buys growers time and greater flexibility with the early applications. But it remains imperative growers maintain their foliar treatments to minimise the risk of virus spread.”

The first foliar treatment, for resistance management, should be from a different chemical group to Actara, he adds.

In-furrow Actara could prove a valuable start to insecticide programmes, Eric Anderson, a potato specialist for Scottish Agronomy, agrees. “If we can reduce the early aphid movement with the in-furrow application, there is a better chance that the subsequent season-long control programme can achieve the desired result.”

Application can be made through equipment developed for in-furrow application of Amistar (azoxystrobin) at 100g/ha.