Hello,

It does look like a mildew fungus, specifically, powdery mildew, which develops on many trees, shrubs, and perennials.  The fungus develops when temperatures rise above sixty degrees. It starts on the underside of the leaves and then spreads to the rest of the plant. 

There are a several ways to manage powdery mildew. Here are some: Don't prune the tree until the end of the growing season. All leaf debris should be picked up off the ground and destroyed or thrown away. Tools should be sanitized with a 10 percent bleach solution in order to avoid cross-contamination. Make sure the tree is receiving full sun, planted correctly, has enough space and is not near rose bushes.  Avoid overhead watering if possible. Treatment with horticultural oils, neem oil, and sulfur are recommended for homeowners. Most fungicide products are only allowed for licensed applicators. The oils cannot be applied when the temps are above ninety and sulfur should only be applied before symptoms appear. Label directions should be followed exactly when applying any kind horticultural treatment.

I have listed the links for several articles below that explain powdery mildew and its management. Please read these and write back if you should have more questions.

Thanks for asking Ask Extension.

Betsy B. - MGV

https://extension.psu.edu/stone-fruit-disease-powdery-mildew-of-cherry-and-plum

http://ipm.ucanr.edu/PMG/PESTNOTES/pn7494.html

https://hgic.clemson.edu/factsheet/powdery-mildew/