Your tree has apple scab (https://www.canr.msu.edu/resources/apple_scab ). There's nothing to treat it at this point in the season. Remove affected leaves from the orchard floor at the end of the season to prevent the spores from infecting the other tree. The time to apply a fungicide (Captan or sulfur) to prevent scab is in the Spring from bud break to petal fall. If you apply a fungicide in the future, be sure to check the label that it is registered for apples and to follow the instructions of the label.