It goes inside of the leaf. Water with soap helps minimally. It was glowing 2 weeks ago.

I've experienced the problem, but I don't have an answer. I think the sap was an issue for only one year, although we've since had the offending tree cut down for other reasons.

Aphid 'honeydew' (not sap) will fall on anything below it, rain or no rain. And while it is sticky, when fresh it washes off easily with a strong stream of water.

Your best course of action is to treat the tree for aphids before this problem occurs or remove the tree.

@lindalana 5b Chicago - I was just recently trying to find a solution to this issue and I found a 14-year-old post on HOUZZ:

Any way to remove Sooty Mold on leaves once it's started?

A post in that thread stopped me looking for solutions - the post by calliopestated:

"I guess I was trying to say...once the mold is on the plant, you don't get it off. You can arrest the growth of it, but short of individually hand-washing each leaf on your linden tree with soapy water and cotton balls, I am saying the black reside is on for the life of the leaf"

My property is surrounded by Liriodendron tulipifera and it seems I have a lot of the sooty mold.

BTW @gardengal48 (PNW Z8/9) commented in that discussion 14 years ago.

Gardengal how to treat tree for aphids?

I have white and red oaks mostly and few hickory. Unless I use something systemic?

I am reading on bringing ladybugs and parasitic wasps but not sure on efficiency.

Cutting trees is not an option. I have had it in previous couple years but mild.

Raymond, am thinking it is "honeydew" and not soot.

" how to treat tree for aphids? " If water pressure is good, a spray nozzle on the garden hose should work, otherwise a systemic, like Bayer BioAdvanced Tree Insect Control will work also.

" am thinking it is "honeydew" and not soot " You would know if it was the sooty mold - it's obvious, as it is black (sooty) and slightly dry and crusty.