The Environment Department (Defra) said ash dieback has been discovered in South West Wales.
It was reportedly found in a newly planted tree on private woodland, but Defra has not given specific details.
It's the first time the Chalara fraxinea fungus has been found in Wales.
A joint statement issued by Defra and the Forestry Commission said the disease has also been confirmed in 115 sites in Sussex, Berkshire, Bedfordshire, Lincolnshire, Yorkshire and Northumberland.
The statement said: "The discovery of the disease in these counties does not mean the disease is spreading rapidly.
"It is likely that the disease has been present in these areas for a number of years, originally caused by spores blown in from mainland Europe."
The disease causes leaf loss and crown dieback and it can lead to tree death.