I have 4 peach trees and one cherry tree. All have a light green fungus on the trunks and the leaves are falling off. The fruit looks ok for now. I've already lost two peach trees. Can't find anything on here about the fungus I have.
Fungus on My Peach and Cherry Tree
Certified GKH Gardening Expert
It actually sounds like you have lichen. The lichen itself does not kill the tree, but they are attracted to trees that are stressed. You can say it is more of a symptom. Here is more information: https://www.gardeningknowhow.com/ornamental/fungus-lichen/treatment-tree-lichen.htm
Yellow leaves on a plant can happen for several reasons. The most common are watering issues, fungus on the leaves or pests. Make sure that you are watering the plant enough and treat the trees with both a fungicide and a pesticide. This should help the trees improve.