My wisteria is 25 years old and flowered again beautifully this year. In the last three weeks the leaves have shrivelled and have all fallen off. I have always watered it when we have no rain but it's looking really sad at the moment. I have trimmed some of the smaller branches and a lot of them do seem to be dead. Nothing has changed in the garden so none of the roots have been cut etc. Any suggestions on what to do now would be welcome.