Uh oh -- Problem with my Japanese Maple! See leaf pics below.
I planted it spring 2016, so it's been in the ground over a year.
We have heavy clay soil. The soil wasn't super amended when I planted it, but I planted it well and kept it watered. No problems last year and this year it's been great until the last 2-3 weeks. I do not currently have mulch on top of the soil.
Thoughts? Overwatering? Underwatering? Something else? The pattern on the leaf seemed distinctive enough.
My first guess would unfortunately be verticillium wilt (Verticillium dahliae). Hopefully somebody else will have better news. :)
Verticillium is very common with maples, and nearly impossible to control, though not always fatal to the tree. It typically starts out with spotty leaf death across the crown, progressing to entire branches dying out as the vascular system clogs up.
If you find a branch that has a fair amount of dieback on it, and cut it as a angle you should find black discoloration in the wood. Control is sometimes possible by cutting off the branch back to where there is unaffected wood. I find it's helpful to sterilize my pruners between cuts so I know I'm not transmitting the fungi. A 1:9 bleach to water ratio works well.
I'll leave you google further info on it.
As a second guess, would be lack of water as it looks like you're on a slope. I often find it's helpful to make a dam on the downhill side and form a basin around the tree, about 18" from the trunk and 2-3" above the crown. That way I have a nice basin to fill with water.
What I would do while you are trying to discover whether a disease is involved is purchase so 3 or 4" PVC pipe. Drill it full of holes. Half inch holes all down the length allowing a foot of undrilled pipe to stand out of the ground. Place 3 or 4 of these pipes that are about 3' long into the soil near your tree at regular intervals to encircle the tree. One pipe a foot away from the trunk, the next 2' away from the trunk and the third 3'. Fill these pipes with water, soak at 3 or 4 times, every other day for 2 weeks. Then once per week until dormancy. Your soil is so dry right now that it is called hydrophobic. Very difficult to get moisture to get near those roots. These pipes get the moisture where it belongs and re hydrates your soil so regular water in the future from above can be accessed by your tree's roots.
