The outbreak is at the Moor House-Upper Teesdale National Nature Reserve in the North Pennines.
P. austrocedrae infection was only confirmed in the UK last year on other trees in this family, at two sites in Scotland.
Forestry Commission scientists who identified the disease believe that it can be transmitted in groundwater, infected plant material and contaminated soil. "This makes further spread difficult to prevent," they said.
The commission's Forest Research agency is undertaking further studies to confirm how damaging it might be on juniper, Lawson cypress and other potential tree hosts.
Natural England, which manages the site, is working with the Forestry Commission and the Food & Environment Research Agency (FERA) to survey the extent of the infection and consider how it might best be contained, as well as putting biosecurity measures in place on the site.
A survey of other areas where juniper is widely distributed will be co-ordinated by FERA and imminent consultation will help to determine a longer-term strategy.