Tree diseases expert witness may opine on tree diseases that contribute to the death of most of the trees in the United States. In An Index of Common Tree Diseases: The Major Disease of Trees in the United States, Steve Nix writes:
1) Chestnut blight is a fungus that has virtually wiped out the American chestnut, as a commercial species, from eastern hardwood forests. Although roots from trees cut or killed many years ago continue to produce sprouts that survive to the sapling stage before being killed, there is no indication that a cure for this disease will be found. The fungus is widespread and continues to survive as a nonlethal parasite on chinkapin, Spanish chestnut, and post oak.
Read more: forestry.about.com.