Evergreen trees do not keep their leaves indefinitely, but rather may grow new ones before the old ones fall, or keep some and drop others over a period of time.
As any southern gardener can tell you, some broadleaf evergreens, like Magnolia grandiflora, drop their heavy leaves in a staggered cycle over the course of the year.
Some, like the live oak, may show a heavy leaf loss in early spring, but retain a few old leaves while the new ones develop.
There is also seasonal needle loss with evergreen species of conifers, like pine, spruce, and fir trees. The leaf drop is a normal process, not a disease. The older,