What is happening to this coral bark japanese maple?

Tacoma, WA

What is happening to this coral bark japanese maple?

Thumbnail by tacomagardener
Contra Costa County, CA(Zone 9b)

Could be sunburn, but it could also be certain plant diseases such as Verticillium.
This is a fungus that compromises the plants' vascular system, and causes sunken dark patches and kills the branches. Ultimately it can kill the whole plant. Not all plants die from it, though. Some trees lose a few branches (usually small ones) but the bulk of the tree still grows. (Cinnamomum camphora is one that does this)

http://www.ipm.ucdavis.edu/PMG/GARDEN/PLANTS/DISEASES/vertwilt.html

Rolesville, NC(Zone 7b)

Where is your trunk flare/root flare? Could the tree be buried too deeply?

Tacoma, WA

Well, I took it back to the nursery where it was purchased and immediately he said frost killed it. With young trees especially maple, they have to be protected in winter. We did have several days that went down in the upper teens..guess that's waay better than verticulum wilt, which he said is rare.

Contra Costa County, CA(Zone 9b)

Glad you got that figured out.
Yes, cold can sure do that, especially if the weather gets suddenly cold when it has been mild for a while.
If the cold comes on slowly, gradually getting just a few degrees colder each day (or night) the plant can adapt. Especially an older, established plant.

Protecting young plants is a good idea. Something as simple as draping a sheet or some plastic over it can help. It is best if you can put up several stakes around the plant to support the plastic away from the leaves or branches. I do this for my Lime which is not hardy in my area. I have a frame of PVC pipe and keep a clear plastic tent over the plant until I know there is no more frost.

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