Are you referring to Tiger Eyes sumac? Sumac is noted for its early loss of leaves in fall, so some of what you're seeing may be the natural fall progression of leaf change. However, it usually develops some reddish color before turning brown and falling off. Yours might be affected by a foliar fungus. Does it come back well the following spring? If so, I'd suggest there's no need to spray for the fungus. As long as the premature defoliation doesn't affect the vigor of the plant, it may not be worth the expense and bother of spraying.