At Arger Fen in Suffolk, staff managing the 119 acre ancient woodland had suspicions of the arrival of ash dieback. It was confirmed that about 35 acres of saplings are infected yesterday.
The Wildlife Trust's site manager for West Suffolk, Will Cranstoun, is responsible for 12 woodlands and he is worried it will be impossible to prevent the spread.
"This site is almost unique in Suffolk as it has a large acre of naturally regenerating ash.
"It is that area which is infected and, if it is wiped out, it will fundamentally change this landscape for hundreds of years to come.
"If it spreads to some of the older ash, we will be losing trees with real history with some of them dating back as much as 300 years."
Experts say little can be done to stop the spread of 'Ash dieback' the disease threatening to devastate the UK's ash trees.
One of the UK's biggest tree growers will seek damages from the government after losing thousands of trees to ash dieback disease.
The disease threatening to wipe out the majority of Britain's ash trees may have spread - as experts say an import ban was too slow.