Q: My wife and I have a cherry tree in our yard that is not doing well. About this time every year the leaves of the cherry begin to show spots, turn yellow, and fall off. The process is gradual as the summer progresses. The tree loses all its leaves by mid-September, only to come back to life in March.
A: Cherry leaf spot is rampant this year. The disease starts with reddish-purple spots on the leaves. Many turn yellow and fall from the tree, leaving an autumn-like appearance in mid-summer. Wet weather favors the disease. It can be controlled with chlorothalonil (click for sources) if sprays begin after flowers fall in spring.
At this point (July) you can spray a couple of times to try to halt disease progression but your major preventative task is to rake and remove all leaves in fall since the fungus overwinters on them. Consider putting fresh mulch on the ground in January to further prevent spread.