Hello group, I keep forgetting to ask this, but finally:

I have this plum tree in my front yard. In recent years some of the branches
have been developing 1cm-long green fingerlike projections on the underside
of the leaves. Is this some sort of fungus? Does this need to be treated,
and if so, how? Is the fruit from those branches still edible?

(Zone 3, Chernozem over a clay bed)
TIA