Hi, it looks like you have two different things happening to your rhododendrons. The white spots on the branches look like Azalea bark scale which also infests rhododendrons. Even though it appears as spots, it is really an insect that has parked itself there and is sucking sap from the plant. They develop a waxy coating so are difficult to control by spraying. If the plants are not too large, you can scrape these off with the edge of a plastic knife or your thumbnail.

Follow the links listed at the bottom of this page from the OSU Pacific Northwest Insect Management Handbook on Azalea bark scale; it will take you to more information on scale insects in the “Common Landscape Pests” section.

Rhododendron leaf notching is caused by Azalea (rhododendron) root weevils. This is a link to the PNW Handbook again. Follow the link at the bottom of the page for more information on control. The leaf damage is caused by the adults feeding. They do no fly which is why they haven’t migrated across the concrete walkway to your other rhododendron. There are several options for controlling them. Prune the branches so none touch the ground. Then you can coat the trunk with a sticky trap substance which you can purchase at your local plant nursery. This will trap the feeding adults. There a couple of cultural controls that will help control the larval stage. See the above link for complete details.

Keep your plants watered and mulched to minimize any environmental stress. Enjoy this gardening season.

I took a fingernail brush to the branches to knock off their covers. Sprayed with insecticidal soap.
I spread diatomaceous earth around the bottom to stop the weevils.
Do you think this will work?
I found more of the Azalea scale on other rhodies and azaleas. They are just a lot healthier and don't have the weevil notching. I'll get to them with the brush and spray too if you think it might work.

The fingernail brush was an excellent idea. Yes, it will work on the other, healthier rhododendrons.

Diatomaceous earth was also an excellent idea. Yes, it will be helpful against the larval stage. It is one tool in an integrated pest management approach that will help control the pests. This coming autumn, place shredded oak leaves as mulch under all your rhododendrons. This encourages birds to scratch through the mulch looking for tasty larvae to eat.

Keep up the great gardening work.