The 42,000 plane trees lining France's historic Canal du Midi are being felled because of a fungus brought to Europe by US soldiers in World War II. Will the planes that decorate the streets of cities such as Paris and London share the same fate?

It's noon in the farmlands outside Toulouse, but the light on the Canal du Midi is dim, almost like twilight.

The deep shade is created by the leafy branches of huge plane trees that tower above both banks, and arch across the water creating a dense canopy.

It's an exquisitely beautiful scene. And one that runs the length of the canal on its 240km (150-mile) course from Toulouse to the port of Sete on the Mediterranean.

It was five years ago that Jacques Noisette realised something was wrong.

"In the spring of 2006, I began to notice that some of the plane trees were dying," says Mr Noisette, who works for the French government agency that manages the canal.

Travels of a tree fungus Image caption Ten thousand boats use the canal every year "This fungus evolved in the eastern USA, where it causes little problem to native species. It was accidentally introduced into Italy in the 1940s, when American troops brought packing materials that had been processed too rapidly for the drying process to eliminate the pathogen. "Over the next 50 years, it spread through Italy. In the 1970s, the disease was found in south-east France. It is now present in Switzerland, Germany and also Greece, where it is killing 'oriental planes' - the only indigenous European plane tree. "As the disease continues to spread, it is likely to eliminate planted planes from the cities of western and northern Europe. At the same time, the spread of the disease in the native range of oriental plane is an ecological disaster, on a par with the loss of elms throughout Europe in the mid-to-late 20th Century." Dr Steve Woodward, tree pathologist, University of Aberdeen

"Their new leaves should have been opening up, but they weren't. We asked ourselves why."

Specialists soon identified the deadly fungus, Ceratocystis platani, for which there is no apparent cure.

"Even a small scratch or cut on a plane tree is enough for the fungus to get inside and attack. It thrives deep in the tree trunk," says Mr Noisette. "Within three to five years the tree is fully infected. There's nothing we can do except cut the trees down, and burn them on the spot."

Researchers say the trees must be felled before they die. Otherwise they could fall on holidaymakers, who travel along the canal in boats - spreading the fungus as they go.

"We know the fungus travels through the canal water," says Mr Noisette. "The trees can get infected when boats scrape up against the roots. Or when uninformed boaters tie-off their boats to the plane trees themselves." (You can see a video shot at the Canal du Midi here).

French agronomist Andre Vigouroux, who has been studying the fungus for years, says it's been traced to the munition boxes American soldiers brought over to Europe in World War II, which were made from North American plane trees.

It's been spreading through Europe, from Italy, and Steve Woodward from the University of Aberdeen, says it is likely eventually to kill the planted planes that line the streets of cities such as London, Paris and Berlin.

"There really are millions of these trees planted in non-native areas.

"We are talking about a massive disaster if the disease continues to spread," he says.

Along the canal tree-felling has begun - 1,000 trees were cut down last winter, 2,000 more will be felled in the coming months.

Image caption The ubiquitous London plane is a cross between the American and Eurasian varieties

It will take years to clear all 42,000 of them, but once the trees have gone, so may the two million tourists who visit the canal every year.

"It is so hot in the Midi that it would just change it totally without the trees," says retired British tourist Graham Barley, who has been travelling up the canal from the Mediterranean in a wooden houseboat.

His wife Linda agrees. "In practical terms, it would make cruising between noon and 4pm unbearable," she says, even in September.

The Canal du Midi wasn't originally designed for pleasure boats.

Completed in 1681, it was designed to link up with another waterway further west - now known as the Canal de Garonne - to allow merchants to move between the Atlantic and the Mediterranean, without having to sail around Spain and Portugal. The sea route was long and arduous, and ships could fall victim to storms and pirates.

Image caption The Canal du Midi planes will take many years to fell and replant

The plane trees were a later addition, planted in the 1830s in part to provide shade to those using the waterway.

Unesco declared the canal a world heritage site in 1996, saying it had "provided the model for the flowering of technology that led directly to the Industrial Revolution and the modern technological age". To retain this status, however, France will have to replant the trees it chops down.

So that's the plan. To replant a variety of trees where the plane trees now stand. Trees with strong root systems to maintain the canal banks, and with thick leaves to create shade.

And ironically, given that American GIs brought the fungus to Europe in the first place, among the new trees to replace the old are 7,000 disease-resistant plane trees - all the way from Mississippi.

Whether colder parts of Europe will be able to use the Mississippi strain to replant any planes they lose in the future, is as yet unknown.

You can hear a radio version of this piece at PRI's The World, a co-production of the BBC World Service, Public Radio International, and WGBH in Boston. The radio report was first broadcast on PRI's The World on 18 October, 2011.