Hi Chuck,

There are many diseases that affect aspen. We have a publication on this: https://extension.colostate.edu/topic-areas/yard-garden/aspen-and-poplar-leaf-spots-2-920/

It looks like Marssonina leaf spot. At this this point in the season, there isn't anything you can do, except try to rake up fallen leaves in the fall and throw in the trash. The fungi overwinter in spores on fallen leaves. While you could spray a fungicide next spring, this disease rarely affects the overall health of aspen and is mostly a cosmetic issue.