TAKE a second look at those virus symptoms. Just because they seem fairly subdued on the foliage doesnt mean there isnt a lot more going on below ground.
Blight may get all the attention from fungicide applications but potato virus Y accounts for 45% of all the disease on UK potato crops, points out Tom Locke, of ADAS Rosemaund.
Until recently, most of the textbook information on PVY was based on studies of King Edward and Majestic but MAFF has asked ADAS and the Central Science Laboratory to find out more about the virus effect on modern varieties.
PVY is most severe in its tuber-borne form, but can also be readily transmitted by aphids. Unlike leaf roll virus which requires the aphid to be stationary for some time sucking sap from the phloem of the plants, PVY is transmitted immediately the aphid inserts its stylet.
The virus is also classified as having two strains PVYo and the supposedly less significant PVYn but Dr Locke says inoculation trials with 12 varieties show differing reactions from each to virus infections. Seed from the trials is being saved and grown on to determine the severity of tuber-borne infection.
In 1996, Maris Piper showed a 36% yield loss to infection by PVYo but only 9% with PVYn. On the other hand, Estima infected with PVYn showed a yield loss of 25%, but only 13% to PVYo. Wilja had losses of about 7% to both strains, while Romano was rarely infected by the inoculations thanks to a high level of resistance to the virus.
Dr Locke warns that relatively minor foliar symptoms may disguise what is really happening to the tubers. "You could think there is nothing wrong with your Estima from a casual look, as the foliage mottling is typically very mild.
"But plant out the infected seed and you can get yield losses of the order of the 30-40% plus that we obtained at the ADAS sites at Terrington and Arthur Rickwood, both with PVYo."
ADAS tests home-saved seed for the virus and has found notable differences in the incidence of virus strains on different cultivars. PVYo is 43 times more common in Picasso than PVYn. Marfona and Estima are also more likely to have PVYo but Pentland Dell is much more likely to suffer from the second strain.
The blanket advice from ADAS is currently not to plant any home-saved main crop seed potatoes with more than 5% virus infection, but Dr Locke hopes it will be possible to tailor this advice more accurately in future to suit the variety involved and the strain of the virus present.
With this in mind, the new MAFF research is also looking at the effect of crop compensation where healthy plants are adjacent to those carrying virus.
If the virus-affected plant is weak, then in theory a higher than normal yield might occur in the neighbouring healthy plant. This is because it has more water, nutrition and light available to it.
However, such compensation does not occur with Estima because of the normal vigour of even the diseased plants.
Potato virus Y costs growers about £30m in lost yield. New research could help to solve the problem reports David Millar.