I agree. It would be difficult to pinpoint the problem without more description, and perhaps some pictures. Is it an inherent disease on the tree or the growing conditions? TreeHelp.com
has an excellent summary of the types of diseases that afflict cherry trees. The two diseases that cause leaf symptoms like yours are black aphids and scales. But fungal diseases are also possibilities. But going back to your description of the soil, poor drainage and waterlogging is always a potential problem. In addition, soil that gets well irrigated on a frequent basis can become depleted of nutrients and essential minerals.
You would need to approach it like a meticulous diagnostician. Start with the leaves, examine them carefully. Look for things that shouldn't be there. What are the leaves like in the spring when they unfurl? And what happens to them through the summer? What are the twigs, and branches like? Are there any swellings that shouldn't be there? Dig a samples of soil at various depth, preferrable on the non-irrigated days, give it a good squeeze or the day after rain. Are you able to squeeze drops of water out of it? And consider having the soil tested if everything else appears in order.