Mineral oils may provide the best chemical control of non-persistent potato viruses in seed crops, two years of trials by Scottish Agronomy suggest.

Records show that the incidence of non-persistent viruses, such as potato virus Y and potato virus A, had been slowly increasing over the past five years, although 80% of seed crops remained virus free, Brian Fenton, a Zoologist at the Scottish Crop Research Institute said in a workshop at the Potato Council’s seed potato conference.

Because those viruses were acquired and transmitted very quickly by non-colonising aphids, it meant they were difficult to control with systemic insecticides, Eric Anderson of Scottish Agronomy told Farmers Weekly.

“Most systemic insecticides can take up to several days to kill aphids, and it only takes about 30 seconds to a minute to transmit these viruses, so they don’t act quickly enough.

“The only chemical group to have any consistent efficacy are the pyrethroids, and in low pressure years they can be effective.”

But in high pressure seasons they could easily be swamped by the numbers of aphids moving over a potato crop, Dr Fenton had explained. “Last season we measured between 300 and 1000 rose-grain aphids in an hour in 10sq m, and over one million in one week in 60sq m. The effect of a weekly spray to control such large numbers is questionable.”

In the Scottish Agronomy trials pyrethroid insecticides had given only 25% reduction in high vector pressure years, Mr Anderson said. Highly refined paraffinic mineral oils, in contrast, had given up to 75% reduction in transmission.

“One hypothesis is it interferes with retention of the virus particle at the end of the aphid’s stylet, effectively acting like a Teflon coating, and making the aphid less efficient at acquiring virus,” he explained.

“We’ve proven they have a positive effect, and they are used widely in northern Europe as part of a virus control programme. But they give no control of persistent viruses, such as potato leaf roll virus, so they need integrating into a control programme with insecticides.”

Cultural measures were also important to minimise non-persistent viruses. “Plant clean seed,” he stressed. “If you plant infected seed then it is easy for aphids to acquire the virus and transmit it to clean plants close to the infection. If you plant 1% virus it could easily increase to 3-4% in daughter progeny over the season even using insecticides in a high pressure season.”

That also highlighted why it was so important to keep early generations of seed clean during multiplication, he noted. “If you get virus in at an early stage it is very difficult to minimise in future generations if insecticides are being swamped.”

Infection could also come from volunteers emerging or from surrounding crops, particularly unprotected ware crops, he added.

“The more you can isolate seed stocks from potential infection sources the better off you will be”.

“It is also a good idea to grow seed entered for Certification in smaller blocks separated by a blank bed, so that if virus is found in one part of a field, not all of the crop is downgraded.”