Are those spots possibly anthrancose?

Could be.

That's what I was thinking. I have a potted acer rubrum (one that is for sale, not that I intend to plant) and it is getting similar spots on some of the leaves. It is still growing much better than the tree above, but I'm afraid if it gets any worse I will have to pull it "off the shelf" and see if it recovers. If not, I guess I'll have to switch out the soil and spray with fungicide (not sure if fungicide helps, but better safe than sorry).

I will try that, but I'm also wondering if it's simply too hot in my climate, or it's getting too much sun.

I'm using a homemade mix. It's a modified version of the 5-1-1 discussed on the Container Gardening forum.

I modifying it with more peat (I use ProMix place of the peat, because it has a wetting agent in it), since the fabric pots I use dry out so quickly.

I assume all ingredients are store-bought as well? No native soil at all? I mix native soil, compost, mulch, and osmocote.

None, but it's the same mix my other trees are growing like gangbusters in.

So maybe it isn't anthrancose. Try moving it out of the sun for a while and see what effect it has.

I don't think A. triflorum is the easiest tree to grow, but you're on to its strong preference for some overhead protection. There's a really nice specimen growing in relative obscurity at the U of Miinesota Landscape Arboretum. It is surrounded by other large maples and is quite content. I'd restrict direct overhed sun and try to find a nursery spot that has some dappled sun for most of the day. A little bit of sun throughout the day: Is that asking too much?

I can move it behind the other trees in my nursery. It'll get dappled sun.

Do you have a permanent spot for it picked out? Mine was moved from my previous residence and it has deteriorated over the past few years. It was about 10' and badly damaged when I hit the trunk with the tractor bucket as I was digging and lifting, opening up a large gash. It's a long story, but this was a very quick and crude transplant. It has produced a heavy seed crop this year, which I'm not so sure is a good thing. I hope I don't lose it, but I've got a sinking feeling it may be on its way out.

I wonder if the wet, cloudy May followed by the hot sunny, dry June contributed to this problem.
Last night Tom Tasselmeyer said it was the 5th driest June in Baltimore; but luckily many of us had a wet May so it isn't really have a huge impact in gardens yet.

You grow a lot of stuff from seed. It's we'll within the realm of possibility you just have an individual at one end of the bell curve.

Based on the pic you provided, I'd be thinking insect feeding issues...but with only one affected? There does appear to be some sun damage as well...as well as some kind of something on what would otherwise be the leader.

Ah the rabbit hole of discussing soil mix ratios and components. Everyone has a preferred, everyone has an opinion. So be it.

So yeah...long way of saying what you already know...this plant has some problems. Doesn't look like the problems are of the kind that would spread to others...if they were, it would have already. Were it mine, there's a chance I'd have already discarded it under the aegis of low end of the bell curve...unless I took it as a personal challenge to eliminate all other possibilities...insect, sun, disease, fertility, soil mix and porosity, etc.

It is being affected by something seems to be hitting most of my maples, something that kills the ends of the branches and then hollows them out in the middle, causing them to die back a few inches.

I thought we had gotten through it, as they all started sending out new, unaffected growth, but now today I noticed the new growth is affected by the same thing. I drenched them with liquid Sevin as I've reached my wits end with whatever this pest is.

The other maples (trident and Japanese) still look a lot healthier, despite being affected by this.

hairmetal4ever

Can you update us on this? I am trying to track down a container version of this now, to try and make it into Bonsai.

Good article here from Dr. Dirr, he makes it sound bulletproof!
Haha......https://www.hmiadvantage.com/three_flower_maple/

Tree died. It never leafed out in 2018.