Hello! 

This is caused by a fungal pathogen called cherry leaf spot. This is a common disease for tarts grown in Michigan and particularly this year with the abundance of rain and early start to the growing season.  

Here are links to some overviews of this disease:

https://www.canr.msu.edu/ipm/diseases/cherry_leaf_spot#gallery

https://ohioline.osu.edu/factsheet/plpath-fru-40 

The fungus attacks the leaves through the stomates of the leaves, so the leaves must be protected all season long. The organic control is the use of copper. Ideally you would apply the copper before rains to prevent infection. Copper applied immediately after a rain helps but you will still get the disease. Most commercial growers apply sprays every 7 to 10 days as the new leaves emerge to protect them. Once shoot growth stops you still need to apply copper but the interval is increased to about 14 days. If you have long dry spells with no rain you can stop applying copper until rain is in the forecast again. This prevents the buildup of copper on the leaves and loss of the leaves due to copper phytotoxicity. Be sure to follow any labels if you decide to use a copper product. 

Preventing infection is the only way to control this disease, so making any applications now when leaves are already infected would not be worthwhile. The best thing you can do for this season is to manage the leaf litter to reduce inoculum of the fungus for next year. I recommend removing and destroying the leaf litter this fall, as mentioned in the links above.