With all your rooftop trees in 3-ft square containers, I suspect the problem is environmental and related to water -- too much or too little. Browning is often from too little.

You don't say how long the trees have been in the same container, or the tree size. Nor what they're planted in. It also helps to know about wind , sun & prevailing humidity.

Hi jean
There not all in 3ft square containers - only my apricot tree is in that size. I suspect it's a watering issue too.
I think too much...recently.
I'd like to know if it will spread to other plants, if I can stop it somehow, if I need to remove the damaged leaves which in fact I've done before only to have it come back again.

This particular tree (I have no idea as to what it is...and when the officer from the parks dept came to check for beetles, he said he thought it was a laurel but wasn't sure) is about 8 ft tall counting the 18" high barrel it's in...which has a 29" diameter.
It dries out quickly so I know it needs to be transplanted.
It's been in this container for about a year. It grew wild on my roof a couple of years ago and I put it in this barrel last season. It needs to be transplanted, but if it's going to be problematic, I might get rid of it instead.
Should I post a picture of the whole tree?

As for the weather conditions recently... very humid and sunny for the past week. The week before it rained everyday.
The soil always seems dry so I assume it needs transplanting.
The black spots really appeared overnight. I looked closely at it last night and it was fine. I did water it heavily in the late afternoon yesterday. not much fertilizer...maybe fetilized it once a few weeks ago.

I'm adding pictures of the tree and the leaves to help with identification.
leaf:
www.mihaus.com/trees/Leaf2_unknownTree.JPG
tree - I cut off almost the entire left side that was totally black. You can see the space there.
www.mihaus.com/trees/unknownTree1.JPG

Don't know where you live, but laurels here have far different leaves.

As for the damage you've shown, not enough for concern.

Okay, next is a rather blunt statement -- go no further if you're in a bad mood.
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- Frankly, I wonder why you're spending effort on a no-name tree when you could be enjoying other decorative or fruiting specimens.

Actually, someone just told me it's a poplar.
Also I actually agree with you about wasting time
with it...but I have a hard time giving up on any living thing.
Also figured out that the leaf disease is fungal and all due to my watering at night...I shouldn't. It shouldn't be wet overnight. Between the new drip system and my night watering is the cause of the problem...

Jean, I thought you might find this interesting. The unknown tree is a Populis Fremontii and is native to the SouthWest and Mexico. How in the world did it end up growing wild on a roof in NYC?

It's also very susceptible to fire blight especially because it's used to a dry climate...and NYC is not a dry climate at all...in fact the opposite.
I have to euthanize it. It also grows to huge heights and needs lots of room for roots. Not appropriate for the roof.
Still would to love to know how it got here,

Well, it doesn't get fireblight. But it does get a different bacterial leaf disease.

I'm pleased you learned how big it can become. Far too large for your conditions!