My forsythia bush is a big sprawling thing. It developed areas where the leaves withered and branches with no leaves. I cut those out, and the near by branches seem healthy and fine. However, the same thing also just happened to our neighbors. They have 4 separate forsythia bushes near to each other (the top looks like one big bush) and two of the four suddenly died over the course of a few weeks. This situation developed rather quickly in both cases...our bushes and our neighbor's bushes, which are on the other side of the property so they are not too near each other. We did identify a 'gaul' on our bushes and were told by Portland Nursery that we should treat it next October after the leaves fall, but that it was benign. We wonder if it is a lack of water over a long period of time? In the case of our buses, they are on a slope and don't get much water in the summer, but in the case of our neighbor's buses, they are right next to their vegetable garden and get more water than our bushes. In one of the photos you can see a dead bush next to one that appears healthy, and the leaves of the one that is mostly dead have turned redish and are whithered and drooping, and most of the leaves have fallen, leaving bare branches.