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I recognized that this year the apples on one of my trees nearly all begin to show marks like the one on the photo.
The marks started as little spots and seem to grow all around the young apples. Maybe it is too early, but I did not find a fitting picture on google.
Can somebody tell me, what this is and maybe what to do about it?
What we are looking at are frost rings - caused by frost during bloom or very shortly after, that was mild enough to not kill the whole flower, yet severe enough to damage some cells. In the growing fruit it causes the growth of cork cells and the discoloration of the skin. The damage comes in various shapes, depending on the time of frost and its severity. Most frequent are rings or partial rings. Sometimes a ring will hamper the growth of the fruit in that area, making the rest swell around the spot during growth.
Unfortunately there's nothing you can do about it now, the damage happened quite a while ago. While the fruit may look less pretty, they can develop mostly normal and are still perfectly safe for consumption. In some cases, the damage will extend under the skin and you will find a woody, stringy area ("cork"), that can simply be cut away before eating or cooking the apple. If you again expect late frosts during apple blossom, you may want to look into protective measures during the critical period.
Further reading and more photos:
https://growingsmallfarms.ces.ncsu.edu/2016/07/frost-ring-on-apples/
https://www.canr.msu.edu/news/southwest_michigan_fruit_update_june_6_2017
http://www.leesorchard.com/2013/03/frost-rings.html
http://www.doorcreekorchard.com/eat-ugly-apples
https://articles.extension.org/pages/66263/apple-skin-russetting
