Woodlands near Bures run by Suffolk Wildlife Trust are infected with the fungal disease responsible for killing off ash trees.
A spokeswoman confirmed the finding on Tuesday after the Forestry Commission carried out analysis on ash saplings with tell-tale signs of the disease, ash dieback, on the 100-acre site.
They are confined to an area called Hullback’s Grove which was coppiced several years ago. It is believed the young ash trees which sprouted on the area have been infected by spores carried on the wind.
The woodlands are part of Arger Fen, near Assington, of which around 40 per cent are ash trees.
Imported nursery ash trees are thought to be responsible for spreading the disease, and a ban on imports was imposed by the Government last week.
The woodland will not be closed but trust staff are asking walkers and visitors to ensure their footwear is cleaned after each visit, so that leaves do not spread to neighbouring sites.
A spokeswoman said: “The Forestry Commission was inundated with samples so it did take longer for it to come back to us but it has now been confirmed we have it, and we are hoping to get more advice later this week.”
It was unlikely woods would close or that any trees would be burned, she said.