The first set of tests into a case of potato ring rot have been completed.
Ring rot is unusual in Western Europe
The disease, which has only been seen in the UK once before, was found in July in a shipment originating from a grower in Herefordshire.
The Department for the Environment, Food and Rural Affairs (Defra) said it had tested 12 stocks of Sante potatoes, with two showing positive for ring rot.
It said the next phase of tests should reveal if the disease has spread beyond supplies already known to be infected.
Defra said the stocks which tested positive were both associated with seed potatoes supplied from the Netherlands for planting in 2003.
The second phase of the sampling will involve the 2004 crop, which has mostly not yet been harvested.
The programme will focus on the farm in Herefordshire as well as three other farms, in Lincolnshire, Norfolk and Newcastle-upon-Tyne, which all received stocks of Sante from the same suppliers in the Netherlands.
Symptoms linked to the disease were first identified on 30 July at a Cambridgeshire packing factory, which has not been named.
Ring rot is mostly found in North America and in northern and eastern Europe.
The UK has experienced only one previous case of the disease, described as the Foot-and-Mouth of the plant world, in Wales in 2003.
Defra said it did not expect to complete the second stage of testing until late autumn.