I have a beautiful wisteria that has been growing successfully for the past 20 years. Suddenly the vines are starting to die. It looks like all of the vines from the old main stock are dieing and only the new vines shooting out from the base are still living. It looks like it isn't getting enough water, but it has just started dieing since we had the week long deluge of rain. Is it possible that the vines could choke themselves? The only maintenance we do is to train the vines through the lattice of the fence. We rarely have to water this plant since it seems to get plenty of water in the location where it is planted (water runs in its direction towards the drain approximately 10 feet away). What can we do to save this plant?