Is France’s Canal du Midi doomed to lose its famous trees?

Trees lining the Canal du Midi are facing a deadly disease with no known cure, threatening to change the landscape of the popular tourist destination.

An increasing number of plane trees along the canal – which line the banks of the UNESCO World Heritage site – need to be felled after being infected with canker stain (chancre coloré). 

Of the 42,000 plane trees lining the canal, over 30,000 have to date been felled due to canker stain.

In October, nine trees along the canal’s banks in Toulouse were felled. 

This is the first time the disease has been found along the Canal within the city limits, but it has appeared on trees in the city before, as early as 2018, with the city engaging in three separate projects to destroy infected trees.

“We cannot predict how the canker disease will develop in the commune,” said the city’s council.

“Water is a major source of contamination. Until now, Toulouse and the surrounding area have been largely spared the spread of canker,” it added. 

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What is canker stain? 

Canker strain is a lethal fungus that appeared in France in the 1940s, which affects plane trees and others in the Platanus genus.

Originating in North America, it is thought to have been brought into France during World War Two on infected wooden pallets sent alongside American soldiers.

Leaves and branches of trees infected with canker stain turn yellow, and the central bark a blue or orange colour as the fungus limits the tree’s ability to ingest sap supply.

There is no treatment for a plane tree that has canker stain except to fell it and incinerate its stem to prevent the fungus spreading. Otherwise trees can remain alive for between a few months and a few years before dying naturally.

There are also no effective control methods to prevent the spread of the disease, such as a fungicide, despite efforts by various agencies to find a biological or chemical treatment.

As the disease spreads rapidly – particularly through waterways as it enters trees via root systems – identifying sick trees and felling them immediately is the only solution. 

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Once a tree is identified as having canker stain it needs to be felled as do all other plane trees in a 35-metre radius as a precautionary measure. 

With around three-quarters of the original plane trees lining the canal succumbing to the disease, there are fears the canal route will look unrecognisable in the coming years. 

Around 19,000 trees have been replanted along the route. However to prevent a further outbreak – and to attempt to protect the remaining plane trees – seven different species have been used, none of which are the original plane tree. 

Whilst this should help keep the canal’s ecosystem alive it will change its look.