I'm not a rhody expert, but I will tell you the "moss" is called lichen, and it's generally harmless. It just uses the branch as a host while grabbing moisture from the air.

As for the leaves, it looks like some sort of disease or fungus. The rhody looks huge and old. Have you pruned it at all? If not, it might bounce back with a good pruning.

 

I did prune it in the spring and it did come back to life. I also fertalized with Holly Tone in May and last week.

I cut out dead/mostly dead branches but I must admit I'm not an expert in pruning.

We love this tree - not only is it beautiful when it blooms, it also provides privacy by blocking our front window. It would be a tradgedy if it dies!

 

I cut out dead/mostly dead branches but I must admit I'm not an expert in pruning.

Look up pruning videos by Cass Turnbull (RIP) of Plant Amnesty. She has excellent guides on how to prune almost anything.

 

Thanks @chrismar I will check her videos out.

I took a few pictures from inside the canopy.

Notice how the healthy branches do not have moss/lichen problems like the affected branch.

My irrigation system was blasting the branch in question and I do believe the harsh horizontal spray played a role. I will avoid running irrigation when possible, but I want to be more proactive to help the tree rebound!

 

Could be this: https://ag.umass.edu/landscape/fact-sheets/rhododendron-leaf-spot

 

Could be this: https://ag.umass.edu/landscape/fact-sheets/rhododendron-leaf-spot

Thanks for the reply, Rob, I appreciate it.

I spoke to one of the guys at SiteOne and he said it's root rot - too much water basically. He recommended a shot of Agri-FOS injected directly into the ground.

Also had an arborist come over and offer to inject fungicide for $150 (not sure which one). Backing off on water helped a lot, in retrospect this problem occurred after watering 5x/day for seeding project.

I'll probably have the arborist do the injection this time, and pay close attention. If it looks like something I can do myself I'll take over next year if needed.

On a side note I also need to treat my ash tree for Emerald Ash Borer (hopefully just as a preventative) :sad:

 

Could be this: https://ag.umass.edu/landscape/fact-sheets/rhododendron-leaf-spot

Thanks for the reply, Rob, I appreciate it.

I spoke to one of the guys at SiteOne and he said it's root rot - too much water basically. He recommended a shot of Agri-FOS injected directly into the ground.

Also had an arborist come over and offer to inject fungicide for $150 (not sure which one). Backing off on water helped a lot, in retrospect this problem occurred after watering 5x/day for seeding project.

I'll probably have the arborist do the injection this time, and pay close attention. If it looks like something I can do myself I'll take over next year if needed.

On a side note I also need to treat my ash tree for Emerald Ash Borer (hopefully just as a preventative) :sad:

No problem! I've had similar issues with my Azalea bonsai trees. They like moist, acidic, well draining soil, but hate having "wet feet". I have had to do chelated iron foliar treatments because my pH was too high and the plant had trouble with iron uptake. Adding peat and pine straw to the soil helped with that.

I have a large Ash tree in my back yard that was starting to lose foliage at the very top due to the Ash Borer. I had it injected this past spring ($250), and it has about 75% of the canopy left. I'd hate to have to take it down, but we'll see how it responds.

Good luck with your Rhody!

 

I'm pretty far off topic here, but there have been a couple mentions of Emerald Ash Borer and the incredibly high cost to treat/prevent. Many of us already own the AI to treat EAB via soil drench (Imidacloprid) in our grub control spray concentrates at higher (~10x) concentrations. Check the price on Adonis 2F or Dominion 2L. Depending on the size of your trees, you can treat for EAB for like $2/tree/year. See https://hort.uwex.edu/articles/homeowner-guide-emerald-ash-borer-insecticide-treatments/ for more info.

 

We are being hit hard here in Missouri with our Ash trees. My friends have 46 acres in the country and they have lost many Ash trees, mostly in the woods, but some in the pasture. That I know of, nothing has been treated as they were 'too far gone'. This is how my friends felt. Seeing a few for myself, I don't see how they could have rebound, no matter what was done for them.