why is the fruit dropping off the tree?

Question

I have a 35 year old navel orange tree. This year, good size fruit keep falling off the tree, not quite ripe. Why?

Answer 1

See the section about fruit drop in this article on citrus trees:
http://edis.ifas.ufl.edu/hs141

This article is about lemons, but it applies to oranges too:
https://www.gardeningknowhow.com/edible/fruits/lemons/lemon-tree-fruit-drop.htm