Is this white fungus on our oak trees harmful?

Thanks to Lisa and Desi Rhoden and Nina Roberts for this great question!

Both noticed a very troubling sight recently at the base of their trees: a white substance that seems to be growing on the bark, and wondered what was causing it and if they could do anything about it.

Both situations appear to involve a fungal organism, or perhaps a slime mold, and are most likely caused by mulch remaining being placed around the trunk. The trees might also be planted a bit too deeply, which is common in most landscapes.

The good news is that the organism here truly does appear to be growing on the bark. It’s clearly not a shelf fungus, which grow out of the heart of the tree, creating larger, more protruding fruiting bodies, and which cannot be treated.

Since the organism is growing on the bark it is may be possible to mitigate the damage and get the tree back on a healthy path. First, push all soil and mulch away from the trunk of the tree until you see the root flare, an area where the trunk flares out, indicating the junction where the trunk ends and the root begins. You’ll want to keep that flare exposed, so that the trunk of the tree remains above ground and exposed to the air, touched by neither soil nor mulch.

When the trunk stays wet, which would have been the case in a rainy spring such as the one leading up to this problem, microbial organisms take advantage of their good fortune and move in for the kill.

But if the area can dry out and remain dry, the fungus will die, and the infected bark will eventually be far enough from the wood that it likely won’t be a problem. Some of the fungus-covered bark may slough or peel off, but don’t be tempted to hurry the process along by removing it, which would expose the inner, more vulnerable wood of the tree to further invasion by other, potentially more harmful, insects and diseases.

After our typical hot, dry summer, this problem should be less noticeable, and probably well on its way to being completely gone.