As well as ash dieback, which could wipe out virtually all the country's native ash trees, there are at least another five imported diseases and pests now afflicting Scottish trees. A further 10, which could pose future dangers north of the Border, have also been detected in England and Wales.
The UK Cabinet Office held an emergency Cobra meeting on Friday to discuss the problem.
So far in 2012, four outbreaks of new tree diseases have been recorded in the UK – more than in any previous year. According to the Government's Forestry Commission, the rocketing number of previously unknown diseases is creating "unprecedented threats".
One Government scientist has warned of a "tidal wave of pathogens" entering the UK.
"It is terrifying," said Martin Ward, a plant health officer at the Food and Environment Research Agency. "It will be very difficult to stop them coming in."
The fungal disease Phytophthora austrocedrae, which kills juniper trees, was found earlier this year in Glen Artney in Perthshire. Ash dieback, or Chalara fraxinea, is now suspected at four sites in Scotland, while sweet chestnut blight and Asian longhorn beetles have infected trees in England.
Other diseases the commission has reported as being found in Scotland are Dothistroma septosporum, which attacks pine needles; Phytophthora ramorum, which kills larch trees; and Phytophthora lateralis, which kills the roots of cypress trees. Pine forests are also being damaged by a lappet moth, Dendrolimus pini.
Five times more tree diseases and pests were reported between 2000 and 2009 than in the 1990s or 1980s. Experts blame the large increase on the expanding international market involving billions of plants.
"Increased global trade and tree imports pose a huge danger to our native trees," said Dr Maggie Keegan, head of policy at the Scottish Wildlife Trust. "All too often novel pathogens end up in nurseries and garden centres and then spread from there into the wild."
This is suspected of happening with ash dieback, and was the case with Phytophthora ramorum, she said.
"Climate change exacerbates the problem as the milder weather means insects, often the carriers of disease, can survive over winter and reproduce more quickly," she added.
Carol Evans, director of Woodland Trust Scotland, said 15 tree diseases and pests were now known to be in the UK. "Ash dieback is just one of many diseases that pose a threat to our native trees," she said.
The trust is calling on the UK government to hold an emergency summit, bringing forestry, plant health and conservation groups together to address the threats to native trees and woods.
Dr Deborah Long, Plantlife Scotland's programme manager, warned of the "devastating" impact of ash dieback and other imported diseases, and called for loopholes in the rules on plant imports to be closed.
Labour's Scottish environment spokeswoman, Claire Baker MSP, criticised the Scottish Government's response to ash dieback.
"The disease was first believed to be in Scotland in July and confirmed in August yet we have had no comments from Scottish ministers until this week," she said. "They must answer questions on when they first became aware of the threat and what, if any, precautionary action they took."
But the Government was defended by the environment minister, Paul Wheelhouse MSP, who attended the Cobra meeting on Friday.
"The Scottish Government has taken swift action to deal with confirmed Chalara cases and will continue to do so," he said.
"We continue to work with the UK Government to find a solution aimed at eradicating this disease.
He said the Scottish Government was working with Forest Research and the commission and had allocated more funds to tree health.