Why is the lemon fruit brown in the center? Some of the skins are brownish in color. It's been a very disappointing tree for 10 years. We bought a Meyer lemon tree this spring and hope it will be a better one.
It would seem as if your trees have what's called "Citrus Brown Rot". This is a rapid spreading disease, and does have treatment, but it is geared more towards prevention. Here is an article that will help: https://www.gardeningknowhow.com/edible/fruits/citrus/citrus-fruit-brown-rot.htm