This vine in Sancerre is suffering from esca, a type of trunk disease.
It's a fungal infection that grows inside the trunk of grapevines, eventually killing the plant. It is more common in older vines, and can spread through a vineyard rapidly. Other than removing infected material and replanting, there is no widely accepted solution for this modern scourge.
However, help may be at hand.
Jonathan Pabiot's hand, in fact. This biodynamic winemaker in Sancerre heard about an entirely natural approach to defeating esca which is remarkably simple - and seemingly very effective. As shown above, the technique involves sawing into the trunk of the plant and opening up the cut. This reveals the fungal infection and by exposing it to the open air, dries it out. Within a year, Pabiot reports that the plant is back to full health.
Incidentally, he believes that the increase in esca has been caused by the use of omega-type grafting which became popular in the last few decades. He believes this method might allow infection to happen at the moment of grafting, but that it takes several years after planting for the effects to be seen.
Pabiot has repeated his solution countless times across his estate and it works every time. He has now started cutting trunks where infection is not yet apparent in the leaves, but which reveals the fungus in the wood, thereby preventing it from spreading at an earlier stage.