I live in southern Ohio & we have numerous maple trees in our yard.

I've been cleaning up branches which fall to the ground for many years. I've noticed this light blue (or white-aqua) substance that grows on these small limbs (shown in pics below) and I'm wondering what it is.

Mold, Mildew, Fungus?

It looks like a mold or mildew or possibly a fungus but I'm not sure.

Can you identify this substance?

Harmful To the Tree?

Is this substance harmful to trees?

Harmful to Humans?

When I break up the sticks to dispose of them the substance is dry and often explodes like a power into the air. I'm wondering if breathing small amounts of that substance is harmful or if even getting it on the skin would be harmful.

light blue substance example 1

light blue substance example 2

light blue substance example 3

  • 5
    It could be lichen. Commented Dec 4, 2022 at 18:53
  • Thank you, I'm taking a look at that. 👍🏽
    – raddevus
    Commented Dec 4, 2022 at 19:06
  • 3
    It is absolutely, 100% lichen.
    – fgysin
    Commented Dec 9, 2022 at 8:56

1 Answer 1


After a comment was posted, I believe that the answer is that this is a lichen known as the common greenshield lichen.

Here's the main image from the wikipedia entry -- which looks very similar to what I'm seeing on these twigs.


The entry also mentions that they grow on the bark of trees.

Unfortunately, the entry does not explain if this has any effect on the tree or humans.

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    – Community Bot
    Commented Dec 4, 2022 at 19:19
  • 6
    In the maritime temperate zone where I live, lichen is extremely common on trees (finding a tree with no lichen would be very surprising here). It's technically a parasite, as it takes water from the bark, but coevolution means that it's not a threat to tree health. I can't imagine any lichen can be harmful to people; in parts of the Arctic, lichen is the main diet of reindeer. Commented Dec 5, 2022 at 10:34
  • 2
    Very few lichens are poisonous. Commented Dec 5, 2022 at 10:36
  • 5
    @TobySpeight, it's not a parasite as it doesn't take from the tree but from water running down the bark on the outside and rain and so on. It just uses the tree as a place to grow and doesn't feed off it. It's not harmful to the tree. In some very old growth trees the lichen can sometime overgrow the entire tree which still isn't harmful in itself but could prevent the growth of new sprouts and branchen etc.
    – Max
    Commented Dec 5, 2022 at 11:23
  • 4
    As for humans: As @TobySpeight said, few lichens are poisonous and I couldn't find any hint that common greenshield lichen are, so I would assume they are not. I'm also from an area where they are common to a point that "thats how trees look". As a kid we always played with sticks like that and no one ever got a skin reaction from something like that. They can't regulate moisture (no roots) so they can dry out completely whichis why the would explode like a powder.
    – Max
    Commented Dec 5, 2022 at 11:37

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