Answered from another post:
do you notice a lot of holes on the trunk or on large branches of the trees similar to the ones on the attached picture? If you do, the problem could be the Linden borer or Asian Long-horned beetle (see attached pictures). If you note that you have the Asian Long-horned beetle in your area, report it to the DNR. Also, if this beetle has caused the death of your tree, it needs to be cut down, chipped and the chips burned. The DNR may do this for you. There is no control for this insect, chemical or other, unfortunately. The damage it causes is comparable to the green ash borer, both of which came from Asia. If you do not see multiple holes nor see this borer, read below for disease information. If you note the Linden borer, click the following link for detailed information and control. http://learningstore.uwex.edu/Assets/pdfs/A3813.pdf
Linden trees do suffer from disease such as leaf spots, cankers, powdery mildew and wood decay. But because your tree is so barren, it may be difficult to identify these. I have include information on these as well.
Multiple fungi can cause leaf spots or leaf blotches on the large leaves of the American linden, including an anthracnose fungus and the leaf spot fungus Cercospora microsera. 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.
Cankers are sunken areas of dead tissue that may appear on an American linden's trunk or branches; they are sometimes very noticeable and other times hardly visible. The leaves on infected branches will turn yellow or brown and wilt; cankers kill branches or stems that they completely girdle. These dead spots are caused by fungi or other factors. To address cankers on branches, prune off the problematic branches as soon as you notice the damage, making any cuts well below the bottom of each canker into healthy tissue and positioning cuts just above stem junctions or next to a branch collar, the area of raised tissue where a branch meets the trunk or a larger branch. Once cankers appear on a tree's trunk, there is no way to address the canker, but providing the tree with excellent cultural care will prolong its attractive life.
American lindens are potentially affected by multiple different fungi responsible for wood decay. These pathogens generally affect the linden through wounds; old or declining trees are most likely to develop wood decay. Wood decay fungi are most easily identified and differentiated by their fruiting bodies, mushrooms or conks, that emerge from near the base of the plant or wounds. Once fruiting bodies are observed, the infection of the tree's wood is likely extensive. Properly prune trees and avoid wounding the American lindens to avoid problems with wood decay fungi. Consider removing declining specimens that could pose a threat to property or life if they are structurally compromised enough to fall.
Powdery mildew is a fungal disease that appears on American lindens and a host of other landscape plants as powdery white growth on leaves and shoots. Emerging new growth may be stunted or distorted, and infected leaves usually drop prematurely. This fungus is dispersed by wind and develops most rapidly in shade and when temperatures are 60 to 80 degrees Fahrenheit. Unlike many other fungi, powdery mildew spores do not require free water to germinate and infect plant tissue. Positioning the American linden where it will receive full sunlight, pruning out areas of dense growth to encourage good air circulation, and avoiding excessive nitrogen fertilizer applications will minimize problems with this disease. Dennis M. Replied July 1