This is a heavy population of scale insects, specifically White Prunicola Scale, which commonly infests cherry trees and their cherrylaurel shrub relatives. Scale can more easily overwhelm plants that are stressed, though that is not a necessary precursor to being colonized by the scale. Populations tend to take several years to build up to this level, so they were probably present before being noticed. Information about the life cycle of this particular scale can be found in the linked page, and management options (which tend to involve one or more insecticides) can be found in our Introduction to Scale Insects page.

We're guessing the yellow leaf spot you mention is Cherry Shot Hole, a very common disease among Yoshino and other flowering cherry varieties, especially in years with wetter spring weather.

Aside from any scale treatment you wish to try, just make sure the tree is being monitored for watering needs during this ongoing drought so its stress level is minimized.

Miri

Thank you!

You're welcome!