Q.Is My Flowering Cherry Tree Dying?
The leaves on our young cherry tree have suddenly turned brown within three days! The tree is supposed to be a fruitless variety. It has been a relatively wet summer here in Gainesville, Georgia. The tree is in a well drained area near the top of a hilly yard. The trunk of the tree appears to be in good shape. I scraped a bit of the bark off and the trunk has a very green and healthy appearance. The main trunk of the tree is about 4″ in diameter and it is approximately 12 ft in height.

Certified GKH Gardening Expert
This looks like the start of Fireblight! This article will help:
https://www.gardeningknowhow.com/plant-problems/disease/fire-blight-remedies-and-symptoms.htm