This is a bitter-sweet update. Get a box of tissues ready.

Liz and I have finally made it to the famous Canal du Midi in southern France - that 17th century engineering marvel that links the Atlantic with the Med. You might have seen it if you watched chef Rick Stein on the telly travel on a hotel boat along the waterway.

Anyway, there are a number of reasons why it's famous, not least of which is that it ranks among those achievements - such as heavier-than-air flight, flying to the moon, and saying "b******s" on the BBC - that came under the heading, 'It Can't Be Done.'

Monsieur Pierre-Paul Riquet, was one of those who turned his back on the nay-sayers in the late 1600s and persuaded the government of the time that oui, indeed the canal could be built because he had solved all the problems in contructing it. Long story short is that he was given the green light, and despite constant criticism from his opponents he went ahead and built it, introducing some of the best working conditions in the world at the same time.

The sad thing is he died just eight monthhs before it opened (Kleenex number one).

Since then the canal has been used for commerce and tourism, and today is extremely popular with pleasure boaters, winding its way as it does from near Sete on the Med to Bordeaux. Its meandering course, taking it through some of the nicest places in the south of France is another reason for its popularity. There are quaint bridges, many many locks - including the celebrated staircase lock at Fonserannes - and it passes through numerous historic towns and villages.

But perhaps its most celebrated aspect is its plane trees, which, for the past 200 years or so, have lined the canal, lending a tenacious grip to the banks, preventing erosion, while at the same time creating a green tunnel of love along which your boat glides in dappled splendour.

Picture postcard-perfect, the combination of arboreal splendour and engineering excellence have gained the Canal du Midi UNESCO World Heritage status, and have done the French postcard industry no harm either.

It's estimated there are something like 42,000 plane trees lining the banks of the Midi. In summer they are undeniably gorgeous; in autumn they turn golden; in winter their starkness has a beauty all of its own, and today in springtime as we cruise along, the new leaves glow in the sunshine.

But it's all about to come to an unbelievable, unbearable and incredibly sad end, because they've started to cut down the whole blooming lot. Every tree is, apparently, for the chainsaw, the result of a deadly fungus that has been attacking the plane trees for the past five years or so. Time for Kleenex number two.

As Liz and I sailed toward Colombiers yesterday afternoon, celebrating having survived the fiercesome staircase lock at Fonterannes, our good mood was shattered when we came across the waterway equivalent of road cones - floating orange buoys cordonning off a work area. On the banks we could see fresh stumps, where the plane trees had been felled, some showing gaping holes in their trunks where, presumably, the fungal disease had taken its toll. Around these stumps sawdust lay, the blood at the crime scene, except in this case there is no culprit to be caught, no murderer to be hanged.

This morning we were looking at postcards of places on the canal, and recognised a statue of a water-bearer at Villeneuve les Beziers where we'd stayed the previous night. What we didn't recognise was all the lovely plane trees in the background. They had all gone. Not only that, the stumps had been removed, which is why we saw no evidence of the trees when we were there. I actually felt a lump in my throat, and even as I write this my eyes are stinging.

I guess we are lucky in that the cull is still in its early days, and we have already sailed along some leafy watery avenues, with more to come. But soon the plane trees of the Midi will be no more, and it will be years before the new saplings mature enough to be postcard material.

I don't know how to finish this. I need Kleenex number three.