i would personaly and its the best time of year thow im know expert im sure someone more knowladgable will tell you better excuse my spelling .my ornimentle grapevine had one long part that litteraly seamed to die almost back to the main stem as if it had been poisaned . this was about 6 years ago . now youd never know and its fine touch wood (ie my head lol) .

A photo would help to identify whether this is powdery mildew or not. If you want the vine to survive this is not the right time of year to cut as it will bleed sap. Pruning, at at 72 foot it needs pruning, should be done in the middle of the winter when the sap is all down in the roots.

I would get rid of it as soon as you can, and spray the newer one with Bordeaux mixture (copper sulphate) if you can get hold of any (unlikely), or a similar fungicide. You could always try taking rootstock cuttings from the old one before you burn it, and immerse them in a fungicide solution before planting them (or dip in a fungicide powder). It depends on how cold it has been where you are. Here it is still quite chilly, with snow on the ground, and a lot of the vines in the surrounding fields haven't been pruned yet. (That, of course, could just be lazy locals)
Good luck and welcome to GoY! Let us know how you get on.

We've had temperatures as high as 15˚C Gattina

sounds good to me gattina better than letting it spread further . i cut mine of in mid summer as it was about to go onto the maine stem . its fine now x x .

You're doing better than us, then, Mg.!

Yup I know :-) And southern England has been warmer! It all depends if Naomi wants to save the vine or just destroy it. It is fairly easy to treat powdery mildew, as you explained. Oh and I think Bordeaux Mix is still available - viewed as organic, which it is not!

I thought it had been withdrawn as from 1st January, but it still seems to be available under other guises. Italians do tend to ignore inconvenient rules, though.

As do Spaniards! By mid-April any new vegetable plants are liberally sprinkled with copper sulphate - I didn't even know about its withdrawal! I bet I will see boxes of it walking off the shelves in our local ferreteria! ;o)

Here it just goes under the local name, Poltiglia, which is copper sulphate and lime mixed, but now, it seems, you have to ask for it specifically, you won't find it on the shelves.

I wish you could see one of our (3) ferreterias, Gattina. It's exactly like the 'Four Candles' shop of the Two Ronnies - little cardboard boxes piled up on the ceiling-high shelves and the owner can go to any one of them and know exactly what's in it!

Ours sound very similar to yours, but have attics and yards and cellars, too, stuffed to the rafters. How do they remember it all? Once needed a big, lidded bucket and we climbed so many sets of winding stairs to get to where they were kept, I kept thinking "Heaven help them come stock-taking day!" Funny, though, whenever you need something fairly mundane like heavy-gauge, plastic covered wire, or upholstery tacks, they never seem to have it.

Lol!