It is probably a fungal disease called anthracnose, also common on other trees such as maple, oak and ash.  During years of extended cool, wet springs, symptoms appear as water soaked lesions on the leaves.  These spots enlarge and turn reddish-brown or black along the veins.  Cultural controls are only marginally effective for anthracnose and are aimed at raking up and destroying fallen leaves in the fall and removing infected branches in spring.  Maintain trees’ healthy condition by adequate water and fertilizers.  Anthracnose is rarely a serious threat to trees’ survival.