While inspecting another problem I observed that most of the leaves are having two red dots (or very small bulbs, maybe cocoons) at the stems of their leaves: it is something harmful or even a pest? If so, do I need to do something?
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I turn the floor over to Mr. Chas. Darwin:
“Certain plants excrete sweet juice, apparently for the sake of eliminating something injurious from the sap; this is effected, for instance, by glands at the base of the stipules in some Leguminosæ, and at the backs of the leaves of the common laurel. This juice, though small in quantity, is greedily sought by insects; but their visits do not in any way benefit the plant.”
Cherries, too. If you look closely, there will be a pair of staggered glands on the leaf stems; sometimes prominently, like in the species you are growing (Prunus avium aka sweet cherry?), and sometimes far less so, as in the common wild black cherry of North America.
I planted a new cherry tree and it has these small red bumps either side of the stem joint. I have a myriad of insects feeding on them including native bees, flies, ants and ladybirds. At first I thought they were some kind of sap-sucking pest, but research says it's an amazing action by the tree itself to attract beneficial insects.
I found the answer here about 3/4 of the way down this page today on 6/21/15:
It is a defense mechanism of the tree to protect itself from caterpillars. The sweet sap of those nodes attract ants, which defend the food source (the sap) by attacking caterpillars.