What causes the yellow and brown spots on our sour cherry tree?
The tree is most likely sick. Plant diseases take years to develop versus diseases in animals caused by either bacteria, viruses, or parasites. This is because the tree has thick cell walls that can be a huge barrier hindering viral infection. Another thing is that trees lack a circulatory system and rely on partial water pressure from the air to draw water from the roots up to the leaves.
Your tree might have a virus that is slowly killing it. Which insects are endemic to your region? We can deduce the virus if we know the main types of bugs living in your area.