Thank you for attaching the images. They clearly show that your pear tree is infested with pearleaf blister mites, sometimes referred to as pear eriophyid mites. These are very tiny mites, much different from the more common spider mites which you may be familiar with.
Blister mites feed from the undersides of the leaves, beginning very early in the season, just as the leaves begin to unfurl. Their feeding causes reddish to yellowish green blisters which turn brown or black as the tissue dies later in the season. Leaves may drop prematurely.
Bartlett trees are known to be very susceptible. So, I suspect your 100 year-old tree has some degree of natural tolerance/resistance. That said, this spring seems to have been quite favorable to the disease.
Treatment is a late-dormant spray at the pre-pink stage, and a different dormant spray in the fall. Management also includes raking/removing, and discarding, all affected leaves that drop.
Horticultural oil is suggested for the pre-pink stage in spring.
Because the mites survive the winter by hiding beneath the bud scales, applying lime sulfur in the fall can help reduce the populations. However, packaging for home gardeners has been difficult to locate in recent years. You might try local farm stores.
But, because your tree is likely to be quite large, consider hiring a pest control company which sprays fruit trees. You may need to phone a number of pest control companies to locate some which spray edible pears for pearleaf blister. Local farm stores may also be a resource for such companies.
Contact at least 3 companies, compare their comments, then hire the one you feel most comfortable with.