Certified GKH Gardening Expert
This is, likely, a case of just a little too much water! They are quite drought tolerant, though they benefit from watering. They do prefer a thorough dry out between waterings.
Treat with a mild fungicide, and give it a light pruning in Spring. Make sure to let it dry out down to a few inches between waterings, and it will likely recover.
Here are some articles that will help: