Hi, this is Steve and thank you for your question on aspens and sorry you are having issues. First we need to rule out aphids which this weather. Look on the underside of leaves for small - usually green - soft bodied insects. If you find them, please let me know and we will deal with that, though I don't expect you will. It’s always difficult to diagnose specific problems from pictures, but you most likely have a fungus issue since we have had a cold wet spring. I’m including information for you to read over and see if you can self determine the specific fungal infection. Unfortunately, as the information will point out, there is not a great deal a homeowner can do and it will be your decision concerning control by you or from a professional. Unfortunately, aspens are not well adapted to our elevation along the front range. If you would like to have a more precise diagnosis, you may bring an affected branch along with a healthy one on M/W/F from 9-1pm and a Master Gardener will examine them. This is at the Extension Office at Island Grove Park in Greeley. We also have Tree Team which can make visits for a nominal fee. You may schedule that visit by calling during the above hours to

Phone:<personal data hidden>

Best of luck.

Aspen Fungus Diseases  https://extension.colostate.edu/topic-areas/yard-garden/aspen-and-poplar-leaf-spots-2-920/

Hi Steve and thanks so much for your prompt response. We live in Estes Park so the aspen are well adapted here. Last year these trees had sparse leaves too but nothing like the sparseness this year and please see attached picture. We are quite familiar with asphids and although we did not have any aspids last year we still treated just in case. And this year still no aphids anywere on the backside of the wilting and brown spotted leaves.

Can a fungus hang on and grow in two consecutive years in these trees? If yes, is there any treatment?
Thanks, Faith

Hi Faith, yes, a fungus can carry on from year to year in a variety of ways: diseased leaves that are left or even in cracks in the bark. The publication I sent does have some info on control, none of it greatly helpful I am afraid. I'm a Master Gardener in Weld County, most familiar with our altitude, short grass prarie problems. So, I may be missing something. I would suggest you contact the Larimer County Master Gardeners and send them the same pictures and info you have sent thusfar. They may be more familiar with aspen problems and I know several of them and they are very knowlegeable people. Email that info to <personal data hidden>. They have normal shifts where gardeners moniter the email. With that, disregard the information I sent regarding contacting or visiting the office in Greeley. I feel sure the Larimer County gardeners can help and hope it is an easily solution. Best wishes, Steve.