Rhododendron has black trunk and bad leaves
Last summer our rhododendrons and another plant (don’t know the name but a photo of a leaf is attached) began getting dry looking leaves with holes, almost like they were burned. We thought it might be from the sun or heat. But we are worried that it’s a disease that is spreading in our garden. A couple weeks ago we trimmed our largest rhododendron, cutting off a good amount of branches around it’s base. At the time the trunk looked fine, but yesterday I noticed that the trunk and branches are turning black! And our smaller rhododendron has a small amount of the black splotches. Is this a disease? Should we be treating it? Can it spread? Thanks for your help!
Thank you for your question. Your rhododendron appears to have a variety of problems. The most concerning is the trunk, whose darkened bark can be a symptom of phytophora root rot, which will eventually kill the plant. The leaves appear to have leaf scald (from sun) and chlorosis, or a lack of nitrogen that leads to yellowing.
The leaves on the picture on the right appear to be of a different plant, perhaps a Fatsia japonica? They appear to have powdery mildew on them, but it's not closeup enough that I can see whether there are spores there. You'd need to take the leaves into your Extension office so they can examine them under a microscope. Here is an article about white spots on leaves.
You can read about the problems rhododendrons face in this Extension publication. I would suggest you have your plant tested for the phytophora fungus at the OSU Plant Clinic, whose fee is $45. The list of testing they provide is listed here, and their contact information can be found here.
I hope this information is helpful to you. Good luck!