I have 2 live oak trees in my front lawn, they are fairly youn (less than 4 years old) and they have a green mossy substance on the trunks growing on the bark. On one tree it is accompanied by a yellow ring. I see the same thing on a lot of other live oaks, is it harmful? If so what can I do to cure the tree?

If it's a fungus. best bet is to spray a little potassium bicarbonate, or baking soda if that's not available, on the tree trunk.

If it ends up being moss (which would be very unusual on a tree that small that isn't in heavy shade) this won't affect it.

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Shepherd of the Trees
It is not our part to master all the tides of the world, but to do what is in us for the succor of those years wherein we are set, uprooting the evil in the fields we know so that those who live after may have clean earth to till.