Hello,

The yellowing of leaves on your Sour Cherry (Prunus cerasus) tree could be due to a common disease, Cherry Leaf Spot, caused by the fungus Blumeriella jaapii. This disease starts as purple spots on the upper surface of older leaves in early summer, which grow and turn reddish-brown and eventually brown. The affected leaves may develop yellow areas around the spots, die, and fall off the tree. More information on these links:

Cherry leaf spot | UMN Extension

Cherry Disease - Cherry Leaf Spot (psu.edu)

Management of this disease includes:

Sanitation: Rake up and compost fallen cherry leaves to reduce the spore numbers that can overwinter. More information on this link:

Cherry leaf spot | UMN Extension

Fungicide applications: Begin at the bract leaf stage during bloom time and continue throughout the season as needed. Postharvest sprays are also recommended to prevent premature defoliation. More information on this link:

Cherry Disease - Cherry Leaf Spot (psu.edu)

It’s important to control this disease because trees that lose many leaves are more likely to suffer from winter injury, grow slowly the following spring, and produce less fruit the following summer. More information on this link:

Cherry leaf spot | UMN Extension

Before you treat – do one of two options!

TWO (2) OPTIONS

OPTION 1:

For a confirmed and precise identification of the problem, I recommend you email all your images to the MSU Plant & Pest Diagnostics Lab. The experts there can also provide you with the best treatment options. 

Their email is <personal data hidden>. 

Please include your name. The lab will let you know if they need a sample sent. 

FYI, the lab may also ask for additional images.

Their website is at this link:

Plant & Pest Diagnostics (msu.edu)

Their Contact Information is at this link:

Contact Us - Plant & Pest Diagnostics (msu.edu)

This link will explain the details of submitting a sample to MSU Plant & Pest Diagnostics Lab:

Submit Samples - Plant & Pest Diagnostics (msu.edu)

OPTION 2:

You can consult a local certified arborist for a more detailed onsite assessment and personalized advice. To find a certified arborist in your Zip Code, go to this link:

Find an Arborist (treesaregood.org) 

I hope this helps