We cannot make a definitive diagnosis from the information that you have sent, but we would suggest that you remove the affected plant material.  Send it off to the dump.    Water deeply once a week if there is not an inch of rain when the ground is not frozen.  You do not say how frequently you water or how much.  Sumacs generally grow in dry areas and you may be watering too frequently and too shallowly.  This is not an appropriate time of year to be using a fungicide.  Modify your watering regime and your sumacs will probably be just fine next spring.  If there is a problem in the spring, send photos.  vw