What is this white growth on this young walnut tree? Is it a disease or a harmless fungus?
If I remove one, a visible hole is left in the tree.
Other nearby trees do not have this so far (there are 30 trees in the garden). It looks to me like it is growing out from inside the tree. It is present only near the ground. It is not present higher up in the tree.
The tree had leaves before winter, so I do not think it is dead.
Update: The tree has died. No growth, no leaves this spring.
Sclerotinia sclerotinorum, White Mold
Without being able to see much detail, this particular fungus, White Mold may be what you have. It is rare for this fungus to bother mature trees but since your tree is still immature, this is another fungus to consider. Not at all a cool fungus.
If this is a correct ID you have to be incredibly careful about cleaning up and your tree might have to be destroyed. Fun, huh? Read this article before attempting to mess with your tree. Use alcohol on all tools to clean after pruning or doing tests.
Finding this fungus's black, rabbit poop looking 'spoors' inside the tree will be 100% verification. I'd prune a lower branch off and look into the crotch and trunk. These black spoors are 1/4" to 1/2". Not small at all. Clean your pruners. Put all debris in black trash bags; leaves everything.
Hope I am wrong, trust me.
It is a fungus, but it's not harmless - unfortunately, this will have been at work inside your tree for quite a while, digesting the wood. What you see on the outside as these white deposits are the fruiting bodies because the fungus is mature enough now to produce them, and it is a signal that your tree is dying. If it is somewhere it can fall onto someone or something and cause damage, probably best to remove it or at least cut it to the ground.
The white bodies do not magnify well from the picture you've posted, but I'm assuming it's actually Bearded Tooth fungus - check and compare the images shown here https://nature.mdc.mo.gov/discover-nature/field-guide/bearded-tooth
