Its called Dibotryon morbosum, aka Black knot fungus. It is indeed a fungus affecting cherries, plums, apricots. We don't have much of it here in sw Ontario, Canada. But a little further northeast of here, it can be quite prevalent.
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Thank you very much, that was perfect.
Now I know I need to dispose of it so as not to allow it to spread. It is quite limited, but I want it to remain so.