Hi Andrea,

It's a little hard for me to see the insects clearly but I suspect that you are seeing aphids on your aspens. Aphids secrete a sticky substance called honeydew, which sometimes gets colonized by sooty mold, producing a gray coloring on the leaves. Sooty mold is not damaging to the tree and is really just an aesthetic issue. The aphids, on the other hand, suck plant sap from the leaves, which can damage the health of the tree. You can try physically removing them by blasting the tree thoroughly with a stream of water. Conversely, there are insecticide options that can be used to control aphids -- a more thorough explanation of insecticide options are provided here: https://static.colostate.edu/client-files/csfs/pdfs/aphids.pdf.

Feel free to reach out with any additional questions. You're also welcome to send another clearer picture of the aphids to confirm that they are indeed aphids.

Thanks!

Hello - I'm attaching an image of our aspens in winter. The lower branches have a black coating on them.   The rest of the grove looks good - it seems to be only the area near the deck.  We are having Lam Tree Service in Evergreen do some mitigation they recommended within the next month or two.  "Perform soil injection to control insects with systemic insecticide and complete fertilizer on 6 Aspen Stems" Let me know your thoughts please and if there is anything else we should do.  Thanks!!

Hi Andrea,

If you didn't notice any other symptoms (like leaf curling) on your aspens last season (asides from the black coating which is just sooty mold and not damaging to the tree), I would consider holding off and not applying the insecticides just yet. It may be that the population doesn't rebound after the winter to be a problem for your aspens. If you do notice a larger mite infestation and damage to the aspens this upcoming season, then you could consider the insecticide option (the soil injection of systemic insecticide would be the appropriate way to control for the aphids). 

Hania 

Hi Andrea,

No problem at all! Hopefully you don't have a problem with aphids this year! 

All the best,

Hania

Hi Andrea,

Hania has moved to a position outside of Extension, so I am stepping in to pick up this thread!

From what I can see in the photo, it looks like you have a leaf-spot fungus, possibly something like Marssonina leaf spot.  These fungi actually infect the trees in early spring but symptoms are most noticeable later in the summer when it's too late to do anything about it!

They are favored by wet weather, and this has been a banner year for infections across the Front Range.

To help prevent the problem next year, remove and dispose of as many leaves as you can after leaf-drop this fall.  The fungus overwinters as spores in the fallen leaves and will be splashed back into the tree canopies in the spring, so raking and destroying the leaves will help remove the local source of infection.  Fungicides are an option, too; they need to be applied at bud-break to be effective.  More information can be found here: https://extension.colostate.edu/topic-areas/yard-garden/aspen-and-poplar-leaf-spots-2-920/

I hope this is helpful.

Cordially,

John