NIAB head plant pathologist Jane Thomas said while virus levels overall are lower than in some years, a quarter of samples received by NIAB had been infected with PVY, with one in 10 tubers infected at levels high enough to cause serious damage in some varieties.

"The levels are probably a reflection of how well growers were able to get on with their aphicide programmes last season," she said.

Virologist Adrian Fox of Central Science Laboratory, which also offers a virus-testing service, said not all potato varieties are susceptible to the same viruses but recommends testing for PVY and potato leaf roll virus as a minimum.

"It's important growers know the susceptibility of varieties to different viruses. The safest thing to do is to test for all of them," he said.

Thomas advised growers to take soil samples for nematode testing when soil conditions allowed. "The incidence in samples we've seen has shown a noticeable presence of free-living nematodes, which is a worry," she said. "They cause damage by direct feeding on roots."

Other crops, particularly carrots, may also be affected.