Officials have moved onto a farm in mid Wales to remove 20 tons of potatoes infected with ring rot.
The potatoes removed from the farm will be destroyed
The discovery of the disease was confirmed on John Morgan's holding in the Brecon Beacons during an annual survey by Defra.
It was the first time ring rot - described as the potato equivalent of foot-and-mouth - was found in the UK.
Experts say they are confident that the disease was contained at the farm.
The outbreak was confirmed at Middlewood farm in November and it is thought the disease was brought into Wales on infected Dutch seed.
Although ring rot poses no risk to human health, farmers leaders in Wales had said the news was a "massive blow" to the industry.
Annual losses to US potato farmers caused by ring rot have been as high as 50%.
Following a series of tests at the farm, two stocks were found to be infected, but the remaining 20 - all different varieties - were found to be free from the disease.
More than 88,000 potatoes were peeled and cored by Defra inspectors and tested by the Central Science Laboratory - the disease appears as black rings within the potato and the vegetable then rots down.
John Morgan owns the farm in the Brecon Beacons
Officials will now move the infected potatoes from the farm to be disposed of at a landfill site in Aberdare.
They will be buried under two metres of soil and waste.
Mr Morgan, who could face losses of up to £400,000, has been told that no compensation will be available to him from the Welsh Assembly Government.
Transporting the potatoes off the farm for destruction could cost him £25,000 with the disinfection of the farm buildings and machinery costing more again.
Having been warned that his farm could be out of business for four years, Mr Morgan has begun a legal fight over who is to blame and is taking action over a firm believed to have supplied his farm with the seed potatoes infected with ring rot.