Certified GKH Gardening Expert
If they are chalky, it could be the type of water your are using. It may be mineral deposits building up. Watering with a distilled water will stop this. If they are fuzzy, this is mealybug and this article will help you treat it: https://www.gardeningknowhow.com/plant-problems/pests/mealybugs-white-residue-on-plants-leaves.htm
It might also be crown gall, which can stunt the growth of wisteria.