Answer: Check out which leaves are turning yellow. If young leaves near the tips are yellowing while older leaves remain green, your plant is exhibiting iron deficiency. If older leaves are the ones yellowing and dropping, the problem is probably soggy growing conditions which can destroy roots by cutting off their supply of oxygen.
Other causes of yellow, dropping leaves are drought (which doesn't apply lately!), herbicide injury, and physical damage to plant roots. I doubt the whiteflies are the culprit unless you seed lots of them congregating on the wisteria. You would also see their pupae (which resemble yellow to chartreuse, flat scales the size of a typed "o") on the undersides of the leaves.
Thanks for the question!
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