This looks like a fungal disease caused by either Marssonina or Septoria. Once the tree gets infected, there isn't much you can do during the season that they are infected. Fungicides can help to prevent the disease, but don't cure infected leaves. The best time to treat for these diseases is to spray the tree early in the spring before leaves appear. Your trees will most likely survive this year, which will allow you to treat them next spring. At the end of the season, remove infected leaves that have dropped from the tree and dispose of them. Do not compost them or use them as a mulch. Spray with a copper based fungicide that has been labelled for use on aspen trees..