Question: What is causing the brown blotches on the leaves of my plant?
Irregular dark brown marks and blotches, usually with a yellow margin, are a symptom of infection by the fungal disease powdery mildew. On most types of plant this disease produces conspicuous white fungal growth on the leaves, but on wisteria it can be very difficult to see any growth at all on the blotches (without using a microscope).
The disease produces another atypical symptom when it first develops on the leaves in early summer – a pale green or yellowish mottling, which can be mistaken for virus infection. Wisteria can also be affected by viruses so if the problem does not respond to mildew controls and persists from year to year, virus may be involved.