Why are the leaves turning down?
Linden trees are fairly healthy and great trees to plant residentially. They do get some minor diseases that are cosmetic and rarely get a disease that can completely kill them. The only known disease that can kill the linden is called anthracnose. It is a fungus disease that is easily distinguished by black spotting showing up on the leaves first. Eventually it will spread to the trunk and bark of the tree.
These diseases appear as circular or irregular leaf spots that may enlarge, grow together and develop a dark margin as the disease progresses. Affected leaves often drop prematurely. Promptly rake up and remove or destroy fallen leaves and other debris. Where possible, prune out infected twigs when the American linden is dormant and destroy these diseased plant parts.