I am in New Zealand and, therefore, in the middle of summer. I planted two wisterias about 2 years ago. One blue and one white. They have grown well and flowered pretty well. One of them has suddenly gotten shriveled, dried up leaves for no apparent reason. This is the white one. Any ideas what may have caused this?
The sudden shriveling of leaves, especially if there are no spots or discolorations on the leaves, suggests some kind of bug (probably very tiny, since you don't see it,) like thrips or aphids. Or something could be eating the roots. Here's a discussion of the most common wisteria problems: https://www.gardeningknowhow.com/ornamental/vines/wisteria/wisteria-diseases.htm
Probably the best idea would be for you to have some samples of leaf and root analyzed. You can call the nearest botanical garden for information about contacting a master gardener group, who will help you with analysis and treatment. Or you might be able to find advice through a nearby university, nursery, or plant store.