The conservation charity is one of hundreds of landowners that have confirmed the presence of the disease on its land over the past couple of weeks.
Ian Wright, plant health specialist at the National Trust, said: “Unfortunately ash dieback has now been found at Watendlath in the NorthLakes.
“This is the first of several sites where suspected cases have been found on Trust land over the past couple of weeks, with the others mainly in the east and south-east of the country.
“Our tree and woodland experts across the country have been working closely with the Food & Environment Research Agency (Fera) to survey the country looking for signs of the disease, and sadly, it was only a matter of time before we had a case confirmed.
“As a precautionary measure, we started to remove and destroy the 1,000 new plantings, which were less than a metre tall, ahead of diagnosis, to try and safeguard nearby veteran ash pollards – some of which are over 400 years old.
“The ash trees will be replaced with other species, but our main objective is to do everything possible to try to protect as many of the ash trees as we can in the woods, parks, gardens and farmland that we care for.”
The National Trust cares for 25,000 hectares (61,776 acres) of woodland and forest throughout England, Wales and Northern Ireland with many special places, beautiful woods and ancient trees at risk because of this disease.
Mr Wright added: “Many of our wooded landscapes may well be dramatically changed by this disease, and we are particularly concerned about the loss of the some of the hundreds of veteran ash trees that we have in our woods and parkland.
“We will continue to implement best practice, as advised by The Forestry Commission and Fera.
“All the places we look after remain open to the public as normal, but like other landowners we’re advising visitors to follow some simple steps that may help reduce spread of the disease.
“This includes keeping to marked paths when walking through woodland and cleaning mud and leaves from footwear and bike tyres after visiting the countryside.
“Thankfully during the winter and spring the spores are least likely to spread, so we have some breathing space. We are investigating whether there is any other action we can take to limit the spread next spring or whether there are ways of increasing the resilience of our trees.“