Following laborious tests, experts have finished testing stocks at the mid Wales farm where an outbreak of potato ring rot was detected.
The ring rot was identified during a Defra inspection
Two stocks were found to be infected, but the remaining 20 - all different varieties - were found to be free from the disease.
The discovery of the devastating potato disease was confirmed on John Morgan's Middlewood Farm in Bwlch in the Brecon Beacons, during an annual survey by Defra in November.
It was the first time the disease - described as the potato equivalent of foot-and-mouth - was found in the UK.
Mr Morgan has been told that no compensation will be available to him from the Welsh Assembly Government and last week it emerged he was taking legal action against the Dutch firm which supplied him.
During the past fortnight, more than 88,000 potatoes have been peeled and cored by Defra inspectors and tested by the Central Science Laboratory.
Stocks of the same variety as the infected stock were grown at three
other farms, but they were tested and found to be free from ring rot.
Stocks of another variety which were supplied to potato-growers by Middlewood Farm have also been tested and found to be free from ring rot.
However, because of their link to the outbreak farm, these stocks have been designated as "probably contaminated" - which means that they cannot be used as seed potatoes.
As a further precautionary measure, stocks grown from seed potatoes that were
grown in 2002 on the outbreak farm are now being sampled and tested.
Although ring rot poses no risk to human health, it devastates stocks.
Farmers are also worried about the effect on the seed market if the UK loses its disease-free status.
Potato ring rot is a disease which is widespread in crops across Europe, having been identified in recent years in France, the Netherlands and Denmark.
The disease appears as black rings within the potato and the vegetable then rots down.