Unfortunately we don't think the tree can recover from this extreme degree of leaf browning and shedding, though it's hard to tell what originally caused the problem at this point. Is there any trunk damage visible, or were the roots of the tree damaged from any digging (planting, utility work, sidewalk repair, etc.)? Either scenario could explain the drastic damage because it would deprive the canopy of enough water.

Rapid defoliation during extreme heat while the fruit is maturing (enlarging, which uses-up lots of water) suggests that this was a weakened tree going into last winter, and that it did not have the energy reserves needed to maintain normal growth for this entire growing season. Causes of weakening could have been prior fungal canker or Fire Blight infections. Unless this is a variety highly resistant to Fire Blight, that bacterial disease can be quite common on pears. We have seen bad outbreaks of powdery mildew on ornamental pears earlier this season, though this tree' symptoms don't quite seem to match that disease.

All that can be done for now is to monitor the tree for watering needs, if you want to see if it remains viable long enough to ripen the fruit. There is no fungicide or other remedy that will cure its condition or reverse existing damage. If it seems to recover with a bit of new growth soon, or what looks like a normal start to spring growth next year, then consider treating it regularly to prevent the typical pest and disease issues common to fruiting pear, which can be explored in the linked page.

Miri

Thank you for your reply.  There was no excavation or disturbing/damaging of the trunk or roots of any kind.

Hello Galen,

Thank you for the additional information and photos. The lichen is harmless, and may appear more abundantly on branch bark that is receiving more sunlight and rain, due to the tree's foliage loss. Removing the pears still attached to the tree might be more trouble than it's worth, given the tree's large size (for a fruiting pear, which is usually more dwarf), though if it were a younger tree, that might be a suitable approach to spare it the taxing resources of ripening fruit. In this case, though, that's probably not a significant detriment given the other, likely longer-term history of stress that led to this degree of canopy thinning.

Don't prune in the fall, though any dead wood can be removed at any time so it doesn't fall off in strong winds and damage live branches or strip of trunk bark in the process. A certified arborist or licensed tree expert can help evaluate the tree's branching and make recommendations, plus try to make a diagnosis for decline, though not all arborists have extensive experience with fruit trees, as they tend to be pruned differently than ornamental trees. (Pruning for tree health reasons isn't different, but pruning for tree structure that supports the best fruiting is.)

For now, just irrigating the tree's root zone as needed based on rainfall (or lack thereof) is the primary recourse. A reduction if leaf loss suggests that a leaf infection, if it was responsible for the earlier browning and shedding, is abating for the season.

Miri