Rosemary bush turning brown in big sections
This bush has been in a large stock waterer with deep soil and good drainage for at least five years. It faces east for full morning sun, is in zone 8, protected from strong north winds, and gets primarily rainwater except in drought conditions when we do water it. These past few months, it has developed large dead sections, brown dry leaves, so that now about half the plant is gone. As far as we can tell, nothing has changed. It shares the tank with oregano and a chive plant, all of which are thriving. The only other plant around it is a very large pecan tree that does shed its seed clusters in that area. Any ideas what is causing this or what to do?
The pecan tree - unless it's new - shouldn't be causing any stress, and since you haven't moved the plant for several years, it may be showing the early signs of root rot, especially if you've had more rain than usual this season. This article may help you: https://www.gardeningknowhow.com/edible/herbs/rosemary/brown-rosemary-plants.htm
You can also do some light pruning to see if that helps: https://www.gardeningknowhow.com/edible/herbs/rosemary/rejuvenation-of-rosemary.htm