Often by middle of summer deciduous tree leaves can look pretty beat up from various insects and leaf fungi. Extended periods of wet leaves, can allow various fungi to get started. In general, most of these are not lethal, are actually more a cosmetic nuisance. You can avoid further infection in part by moving the forsythia to a location with better air movement. If it was on the ground, put it up on a bench. If the bench is crowded, try creating space between the pots to allow air flow. Usually more direct sun will help keep fungus at bay. None of these environmental cures will fix the damaged leaves, but they should help prevent others from getting bad.