If your trees have lost all of their leaves but there are still patches of green leaves on them, they are most likely under attack by mistletoe.
Mistletoe is a parasite that slowly kills trees.
How It Spreads: The mistletoe is dependent on birds dispersing its seeds. Birds feed on the fruits (drupes) of the mistletoe. The seeds are sticky and attach themselves to the birds. The birds dislodge the seeds by rubbing against other trees. Mistletoe is not very host specific and can be seen growing on a wide variety of trees and shrubs.
Mistletoe with leaves are considered to be only partial parasites because they manufacture their own carbohydrates through photosynthesis, but they still depend on host trees for water, minerals, protein, inorganic ions, sugars, amino acids, and hormones from the tree’s xylem and phloem.