Spots on leafs of wisteria
The spots (stippling) on the leaves of your plant are a symptom of insect feeding, possibly from leafhoppers or mites. This is not powdery mildew.
The following pages have information on these insects, what to look for, and what you can do to manage them.
Be sure to keep your plant well watered during hot, dry periods, to help its roots get well established. Plants that are stressed in conditions that are too dry/too hot can become more susceptible to inspect problems. Spider mites in particular like hot, dry conditions.