Dead leaves, shriveled fruit on lemon tree

[Jonathan Problemtree]

This post is about one of my three lemon trees. Tree #1 has always been healthy, #2 was having a hard time but is in great shape now, but #3 is having a difficult time.

Tree #3 has leaves only on the outer branches and these leaves are small. It produces fruit, but these fruit are small. In addition, it has a couple of branches with dead leaves and shriveled fruit (pictured).

Does anyone know what causes this? (the dead leaves/shriveled fruit in particular)