Has anything changed in its vicinity such as change in lawn chemicals, repaving in the last year, unusual weather, etc.?

Good ideas, but I don't think any of the above are a likely cause. No repaving in our neighborhood for at least 2 summers, we don't use pesticides (our neighbors do, but I don't know if the chemicals would drift that far. All of their trees and shrubs are fine, too). We haven't had unusual weather this year -- not too wet, not too dry, no hot days yet. We aren't able to spot this issue on any trees in the neighborhood (with the exception of ours). :-(

Great idea.....thank you!

Is that all on the same branch? If so something may be wrong with it. Any visible defects?

If that is the north side of the tree it may be due to it being the least productive branch as in it gets the least sun.

No repaving in our neighborhood for at least 2 summers,\\

==>>> what was done 2 years ago ??? .. any signiificant digging or root disturbance???

pic of bark.. and leaf.. and we can ID ... whats growing under it ... water sprouts or other plant ...

what was ambient weather for last year ... we just just out of 8 weeks of extreme drought ... last two weeks with extreme heat ...

all that said.. im not much impressed by the proportion of the plant affected ...

ken

Sometimes individual branches turn color early when there is scale present on them. I've seen oystershell scale on a number of maples in this area (including Autumn Blaze and Redpointe), and infestations often begin on isolated branches.

Based on some google research, it sure looks like "black tar fungus" to me?

Hard to tell from the photos, but that looks like it might be maple leaf blister rather than tar spot. Either way, that wouldn't be causing the early color. Something else going on.

I called a certified arborist in 2013, because friends told us we had a back yard full of ash trees, and that they'd all be dead soon because of the Emerald Ash Borer. They pruned our trees for the first time in 2014, and removed one ash tree. In 2015, I asked the same arborist to look at the roots on the silver maple after seeing some odd roots at the base of the tree. We had them try to correct the girdling root that they found. We think our trees were planted in 1997.

I think the important clue for your tree is 20 years.

Maple trees are more prone to girdling roots than other types of trees.

Leaves may be lighter green, scorched, show early fall color, and early leaf drop. Twig or large branch dieback can occur.

My Silver Maple and my neighbor's Autumn Blaze Maple both have the black spots on the underside of the leaves this year. I asked the arborist out of Green Bay if my lasagna garden was causing the black spots, and he said the extremely wet spring combined with a lot of sub-fifty degree days caused it. He also said the badly spotted leaves would fall off all summer & the tree will keep pushing out new leaves. (That is indeed what my tree has been doing.)

Leaves this spring on my silver maple:

Early reddish color on the silver maple (September 20th last year):

My lasagna garden attempt last October 29th:

Improperly mulched and needs crown raising badly.

Any updates on the OP's tree?

AshBecky 5aWI, I would be interested in hearing what options Arborist supplied you on managing your remaining Ash trees. And is Ash a portion of your name, like Ashly? Many Arborist who ended up becoming well intentioned amateurs, only suggesting customers remove their endangered Ash trees. Which goes against ISA Arborist certification policy since temporarily treating Ash trees has now become so dirt cheap and products have such proven success rates. Tree care companies will refund price of treatment if fails and tree needs removal. If spending $600 or more for removal, its less expensive to full dose tree over 10 years. Or if selling property within next decade, protect tree and let next owner deal with decision. If you have several Ash trees, suggest keeping best one or two. Right now protecting an endangered Ash from its species functional extinction, will also save several other ecosystem species which face Co-extinction without their primary source of live Ash. Here is link to my EAB info blog for more Scientific info. https://scottieashseed.wordpress.com/

Just looking at a Blaze maple, could cause portions of crown to change color. (Joke) But seriously, Freemanii Blaze and Marmos crossbred between Silver and Red Maple, have become iconic for early fall color problems. Maybe because presently, almost 99% of trees produced in Nurseries today, arrive with root flares already too deep in rootball. I agree with what others had suggested, it might be a girdling root problem. Another thing that will inevitably shorten your trees lifespan, is that mound of Mulch/soil at base of tree. Here is link to this old house video instructing owners how to perform a "Root flare excavation" around base of tree. https://www.youtube.com/watch?v=fI12XNNqldA 

A root flare void of any extra soil is so important (Mulch turns into soil), that Forest service suggests sweeping young trees root flare clean of any accumulating organic debris each spring. Also check for any flat spots on sides of trunk, especially on side where colorful branch is. Which is sign of girdling root. Hard sad fact, todays mulch mounding fad is expected to claim more lives of trees, than EAB and Dutch Elm disease combined. Also, black tar spot causes no adverse health problems to tree. So you can rule that out. Do you have automatic sprinkler system hit one side of tree? A one sided tree problem, is usually a root problem. And I can see branch above that one also showing some fall color.

From my observations over last 20 years, Blaze maples that show this early fall leaf color, usually retain those leaves until true fall anyways . And seem to have that problem year after year. Since you watch tree year round, you have best chance to make good diagnosis. If tree does not have problem next summer, but does again year after. Document some details each year, like weather conditions etc.. Overwatering or iron deficiency was another possible problem someone else mentioned. I would think chlorosis would show in all of crown, or maybe root on that side of tree encountered some limestone. But usually other trees in same area show same symptoms, such as crown full of yellow leaves.

You may want to check soil PH, just to rule that out. But know if this branch held on to leaves until true fall, its only a visual problem just like black spot is. Also, 20 year maturity point is time when owners learn if tree will live past 40 years. Or will continue to slowly decline. But I presently think, its not that serious. And even if you do remove offending branch, one above it is already showing same thing. Don't let anyone tell you a good pruning will solve things. And I think low branches can be most beautiful thing on a tree, unless shaded out. With all this info I gave you, answer might be in there somewhere...I belong to a committee at Morton Arboretum where horticulturist that invented tree is also from. So hopefully I will have chance to speak with him about your trees colorful problem. And please keep us updated about problem, since there are so many other Blaze maple owners facing same difficulty.