Natural Resources Wales says a large number of Larch trees will have to be felled in Cwmcarn Forest.
It says an aerial survey in May this year revealed widespread infection by Ramorum disease. More than three quarters of the trees in Cwmcarn Forest are Larch.
So a large amount of the forest will need to be felled over the next few years to remove dead or dying trees that have been infected with the fungus-like pathogen Phytophthora ramorum, which causes the disease.
NRW says the felling is needed because infected trees produce airborne spores that can spread the disease, which is not harmful to humans or animals, to other areas and other tree species.
An environment body in Wales says its taking urgent action after surveys showed that a disease which kills larch trees is spreading. The fungus-like pathogen was discovered in the Afan Valley three years ago and had infected trees in around 3,000 hectares.
Around 1,300 hectares of larch trees have now been cut down in a bid to keep the disease in check and limit the damage. However aerial surveys conducted by Natural Resources Wales reveal it has spread to other parts of Wales, in particular the South.
A further 1,800 hectares are showing signs of infection say experts who are now carrying out ground checks to confirm if the trees are diseased.
John Browne of Natural Resources Wales said, “When we get the test results from our laboratories, we’ll reconsider the scale of these operations and what we need to do to get on top of the disease."
Visitors to woodlands can help reduce the spread of the disease by taking some simple actions such as removing any mud, plant material or leaves from clothing, boots, dogs and car tyres.