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.