Your tree may have cherry leaf spot, caused by the fungus Blumeriella jaapii. Cherry leaf spot targets the foliage of cherry trees but can also infect stems and fruit. Small purple spots on the tops of leaves are generally the first symptom of this fungal disease. The spots turn brown and begin to enlarge and collapse, leaving a hole in the foliage. Older leaves will yellow and drop from the branches, and the cherry tree can lose all its leaves, which weakens the fruit tree and increases its susceptibility to cold damage. Be sure to clean up all of the fallen leaves and dispose of in your garbage.
Properly timed applications of fungicide will help prevent cherry leaf spot. Start fungicide preventive sprays at the petal fall stage and repeat every seven days until harvest. Some say way until the new leaves in the spring and others to start treatment now. It would be best to confirm that this is the problem and go from there. This happens a lot in humid weather places with often with overhead watering.
You may want to get a second opinion and take a sample to your local cooperative extension for confirmation. They will also be your best source for treatment of the problem.