Saplings have been uprooted and burned as a precaution against Chalara ash die-back at Knockmountain Wood, near Kilmacolm, Inverclyde, owned by Forestry Commission Scotland (FCS), and at a nursery at Fochabers in Moray.
The disease is suspected at sites at Carrbridge, south of Inverness, and a fourth unidentified location in the south-east, where tests have been carried out to determine if any trees have been infected.
The disease has not been detected yet in mature plantations north of the Border, but woodland managers are increasingly anxious. If the disease infected older trees it could devastate the landscape.
There are no plans to restrict public access to Scotland's forests, although a ban on car parking near any infected mature trees has been mooted in England.
A spokesman for FCS said: "We now have a site in the Carrbridge area which is under strong suspicion. This is a private woodland which was inspected on October 22 and four samples were taken.
"We are still awaiting the results from this. However, we have issued a Statutory Plant Health Notice (SPHN) for the trees to be destroyed and the owner is in full agreement to do this as soon as possible. The SPHN instructs the trees to be removed by end November.
"There are only 25 trees in total that are under suspicion and they are very small – only planted for a year." The Moray nursery had been told to destroy the plants by yesterday.
The spokesman said the FCS investigations there had shown that seeds of Scottish origin had been grown in Holland and it is understood the infection was brought over in this way.
"We do not have any evidence to suggest it was brought over by spores in the air or that it is present in the vicinity of the nursery, following a 1.5km radius survey around the site."
The results are awaited from the fourth site.
Charles Beaumont, spokesman for timber body ConFor, said the issue of public access had been raised in meetings of the outbreak management team, comprising the Forestry Commission, Scottish Natural Heritage, Natural England and external industry representatives including himself. He said the Woodland Trust broached the subject 10 days ago.
"The view of the senior scientists at the time was that there was no need to restrict access on to the different sites, apart from making sure that cars were not parked under mature diseased trees – the reason being that people walking wouldn't spread it very far on their boots, but if it (the infection in spores) fell on to cars which were then driven 200 miles or so, it would be a very different story."
He stressed this would only apply to infected mature woods where the disease has been confirmed, as in East Anglia.