Is this a disease or insect damage
It looks like there are symptoms of fungal and/or bacterial leaf spots. The weather conditions this year have been stressful on different types of plants and we've seen a lot of issues with fungal diseases in particular. At this point in the season, it is not necessary to treat leaf spots. Plants at this time have stored sufficient food reserves and can tolerate some leaf loss and early defoliation. Spraying is not practical and will not reverse the symptoms.
The plant is losing its leaves. Is that normal? I do not want it to die. Would Some fertilzer help? What type of fertilizer is best for this plant?
We're seeing early defoliation in some woody plants due to high fungal disease pressure this year with all the rain we have had. Wisterias are very tough and we think the plant will be fine next spring, even with some leaf loss. If you want to fertilize, we suggest doing a soil test first to see if any nutrients are lacking. You can find soil testing information on our website. https://extension.umd.edu/hgic/soils/soil-testing
Mature trees and shrubs often don't need supplemental fertilizer if they are located near a lawn that gets fertilized. If you find that the soil test indicates a need for fertilizer, the best time to fertilize shrubs is in the fall (late October - early December) or in late winter or early spring between late February and early April.