Although sugar maples are native to Maryland, they grow naturally in the more western parts and cooler sites. It is likely that the symptoms you see are the result of accumulated stress over time. Heat, drought, soil compaction, air pollution are all stress factors. When trees are under environmental stress, they can become more susceptible to disease and insect problems. Botryosphaeria is a common fungal disease. You may not see visible symptoms for some time until a tipping point is reached, then you see overall decline. When you think about replacement trees, we highly recommend diversification. Choose an assortment of different tree species. If any one declines, you are less likely to lose them all.