It is difficult to tell exactly what this is from the photos. Since it is white, you could check what you have against the attached photo of a condition known as scurfy scale. That's about all I've been able to find in my research though it is limited. Here is a link to the U of MN Extension web page listing some diseases of cherries and prunes including scurfy scale. https://apps.extension.umn.edu/garden/diagnose/plant/deciduous/prunus/branchesdiscolored.html

Further links will take you to pages where you can learn more about control. In the case of scale the best time to apply controls is when their juveniles are walking around and haven't formed their protective shells. We recommend using control measures shortly after they hatch so as not to waste chemicals during other times so it pays to know what scale you have and when to apply chemicals.

Please note that when using pesticides, the label is the law.  By reading all of the information on a label, you will know how to apply it safely and for what pests it is designed for.

If you think this is not scurfy scale, I recommend sending a live sample (say a twig with the fungus on it) to the U of MN Extension plant disease clinic for a diagnosis. Here is a link to their web page where you can learn more about their services, prices and how to submit a sample. https://pdc.umn.edu/

Another option is to have a certified arborist evaluate your tree. Here is a link to the U's page on how to find a certified arborist: https://extension.umn.edu/planting-and-growing-guides/how-hire-tree-care-professional

I hope this information helps.