Symptoms linked to the disease were identified on Friday, and subsequent tests for the bacterial infection have proved to be positive. Both the grower and the packing company have yet to be named.
Action has now been taken to prevent the use or movement of any material that may present a risk. A tracing and testing programme has been initiated, which will include an investigation into the origin of the potatoes supplied to the grower in the Herefordshire region.
Another test, as required under European legislation, will be conducted, but Defra says that there is no doubt about the results of the initial tests.
A Defra spokesman told the UK's Independent newspaper that there has been good co-operation with both the grower concerned and the packing company. "Holding action has been taken to prevent the use or movement of material or equipment which may present a risk," he said.
Ring rot is found in parts of North America and the former Soviet Union and is also established in northern and eastern Europe. The bacteria can survive and remain infectious for several years on potato bags, barn walls, machinery and other equipment that has been contaminated, making eradication difficult.
The disease was discovered in the mid-Wales area last October but was contained. Defra says if the disease was to become established, the effect on the seed potato industry would be substantial.