Can you identify what is going on with this wisteria? The image is a cross section from a vine pruned last month (January) in southern Oregon. Notice the distinct vascular development between the xylem layers. Is this a bacteria infection, a wilt?

This vine was pruned while dormant so I can't describe what the foliage looked like. However, the owner did say that the vine is producing far fewer blooms than it has in the past. It is also growing in proximity to an oak tree.