Aspen with Black Bark - Is it Aphids & Sooty Mold or Something Else?
Last spring 2017, the branches and areas of the trunks were turning black but the leaves looked very healthy at that time. Tree diagnosis was either girdling roots or aphids per a couple tree service companies. No treatment at that time was done. Fast forward to end of summer and it appears to be a sooty mold from aphids and honeydew? The black "stuff" was everywhere: on the blue stone patio, the patio umbrella and the hosta underneath the trees. It's unsightly and you don't actually want to sit on the patio. Currently, a year later, more branches are black and leaves not growing as well on those branches. When rub the branches today, my fingers do not have black residue or feel sticky. FYI - I didn't test for stickiness last fall when the black stuff was everywhere. I did try to scrub the blue stone pavers with Dawn but it was hard to remove the black. The leaves look good as of now and there is growth, however, more dead looking black branches without leaves. Attached images shows one tree today with a lot of black build up on the bark and branches, the other two photos are from last fall when the messiest. Do I have several problems going on? Aphids and something else? Do not appear to have any canker issues. I'd like to get to source of problem so can figure out action needed. Should I have someone come and see if I have girdling roots? Is there some way to prevent aphids on large trees? I have more photos if needed.
The maginification of the photos isn't great enough to see if there are aphids or if the black is sooty mold. A 5X hand lens magnifier is strong enough to check for aphids and is probably strong enough to identify if the black is mold.
Here is a poster of aspen problems.
https://static.colostate.edu/client-files/csfs/pdfs/aspen.pdf Comparing your photos to the poster it does not look like aphid damage on the leaves.
If there is honeydew you should see ants eating it.
The spots on your leaves are probably a common fungus, marssonina. Aspen are particularly susceptible if they are living where it is hot and humid; they have fewer problems when they live at their native higher altitude. One of the best ways to manage that is with good sanitation in the fall; throw the leaves in the trash so the fungus does not overwinter and start up again the next year. More information on that: http://extension.colostate.edu/docs/pubs/garden/02920.pdf
One needs a root exam to tell if the problem is due to stem girdling roots.
I hope the fact sheets are useful. I would try to cure one problem at a time.