I work on an estate that has about twenty-four large wisterias trained over stone columns, etc. I have noticed that three or four plants are shedding their leaves. These are brown and crumpled compared with the glossy green leaves of other plants. Once the leaves fall, parts of the infected plants' structure look very brittle and dead. Whatever is affecting these plants is happening quickly. I would imagine these plants are quite old, as they stand perhaps ten to twelve feet in height. Soil is very sandy, location Surrey. All unaffected plants look good for time of year.
Certified GKH Gardening Expert
Most likely, your wisteria is affected by aphids, which suck plant juices and essential nutrients. Their continual feeding weakens plant growth. Infested wisteria leaves may turn yellow or brown and wilt, curl or pucker. Spray the insects with insecticidal soap or neem oil every 3 to 5 days. In addition, here is an article that you may find helpful: https://www.gardeningknowhow.com/plant-problems/pests/homemade-aphid-control-a-natural-way-to-kill-aphids.htm