Thanks for your question about your aspen trees, Maggie.  I'm not sure which pictures you're looking at online, but your trees' leaves appear to be infected with the Marsso nina leaf spot, caused by one of several fungus subspecies.  Fungi are prevalent in cool, wet springs, and manifest themselves once the trees leaf out.  The fungi have over-wintered in fallen leaves which, if not raked, reinfect the trees each year.  Here's a link to an OSU manual page on the fungus.  You will note that, at the end of the article, there are chemical controls available but they are not available to the home gardener (or there would be an H after them).  So, there's nothing you can do this year, except rake the leaves up and burn them at the end of the season.

Sorry I can't be more helpful.   Good luck!