The root of the problem: Britain's trees are under threat from a disease that has a dark history

What do 19th-century Irish potatoes and 21st-century British larch trees have in common? A little more than you might think : a variant of the pathogen that devastated the Irish tubers, and caused the potato famine, is now attacking large swaths of Britain's larch tree population.

Phytophthora ramorum, as the disease is officially known, was first detected in a small number of oak trees in Sussex in 2003, but was successfully contained. Since then, however, the pathogen has mutated into a new, more aggressive form, and attacked the commercially important larch forests of south-west England and Wales: "We currently believe there are about four million trees with the disease, all of which will have to be felled," Roger Coppock, head of specialist advisers at the Forestry Commission, says. "It's a worrying situation to be in."

Extremely worrying when you consider that there is currently no cure for the disease. "The arrival and rapid spread of the pathogen means we are entering a new era of forest management: biosecurity measures such as boot-cleaning and temporary limits on access are likely to be the norm from now on – it's unlikely that we'll be able to return to the place we were at prior to the outbreak," Coppock says.

The effects are all too clear to see in forests in southern Wales, where some 150 of the 800 hectares infected have been felled, leaving vast areas of "scorched earth" on hillsides and forests that are off-limits to hikers. And things are likely to get worse before they get better. Unlike other conifers, the larch sheds its needles in winter, so the telltale signs of the disease – shrivelling and changes in bark texture – are not immediately apparent, so many more trees than currently thought could be infected.

One problem is the damp winter weather. As the disease is largely waterborne – being transported in rivers and streams, on walking boots and even in the mist – the wet weather is hampering containment efforts.

"The heightened threat requires us to take a strategic approach to our forest, and we have recently formed a Biosecurity Programme Board, with members drawn from the Government, private and voluntary sectors, to formulate a new, more careful approach to our forests. The European Union is also reviewing its biosecurity regime," Coppock says.

And as larches make up 5 per cent of Britain's forests, we as a nation are going to have to review the way we think about our woodland and get used to restrictions. We may even have to change our collective gardening habits, as although the experts are baffled as to where exactly this particularly virulent form of the ramorum pathogen came from, it is widely thought to have arrived on British shores via the trade in live plants. So when you're filling your garden's borders and planting your beds in the forthcoming months, plump for an English rose or a British pansy or two – the larch may thank you for it.