Some Major Stems Are Dying.

Question

Four or five major stems are dying back. This is on the west side of the house in full afternoon sun. Plant is at least 40 years old.

Answer 1

Unfortunately, the type of plant isn't mentioned. It could be that it is, simply, at the end of its lifespan. In the case of infection, it could need a fungicide.

https://www.gardeningknowhow.com/garden-how-to/info/using-fungicides-in-garden.htm