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What kind of mushroom/fungus is the one shown in the photos?

Description: Looks similar in size (~4-5 cm as I remember) and shape to a puffball. It is not hard to the touch, unlike typical mushrooms that grow on trees. Several balls were growing individually (not in clusters) from the 2-3 m tall stump of a broken tree, at multiple locations from the bottom to the top. They were breaking through the bark from the inside.

Time and location: Saxony, Germany, in a mixed forest. Early September.

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    $\begingroup$ Very interesting! Do you happen to know the species of tree this stump is from? Knowing the "host" can be important for identifying fungi. The leaves in the background look like they could be from European beech, but the bark doesn't look right to me ... $\endgroup$
    – tyersome
    Sep 9, 2020 at 18:23
  • $\begingroup$ @tyersome Yes, the background is beech. This tree I don't know, and I can't tell from the photo. If I get the chance to go back this week, I'll take a closer look and will try to figure it out. Maybe by then the mushroom fruiting bodies will have developed enough to give us some more hints. $\endgroup$
    – Szabolcs
    Sep 9, 2020 at 19:28
  • $\begingroup$ Most trees are beech in this forest, but there's also plenty of oak, birch, conifers, and other things. $\endgroup$
    – Szabolcs
    Sep 9, 2020 at 19:47
  • $\begingroup$ they look like mosaic puffball mushrooms (Lycoperdon utriforme), which do grow in Germany, but they don't grow on trees :-/ I don't know what these are $\endgroup$
    – JimN
    Sep 9, 2020 at 20:03

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I believe these are Fomitopsis betulina (= Piptoporus betulinus), the Birch Conk (or Birch Polypore or Tinder Polypore), a common mushroom throughout the Northern hemisphere.

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Fig. 1. Young Fomitopsis betulina emerging from a downed birch log. image: Eric (CC BY-SA 3.0)

The diagnostic features here are the scattered fruiting on dead wood, the off-white, crackling surface, and the shape. F. betulina is not a puffball, but a polypore like other shelf fungi that grow off the sides of dead trees. Unlike other shelf fungi, they remain fairly soft. These young specimens resemble puffballs, but will expand to expose the pores on their underside. Read about F. betulina and see images of mature mushrooms here and here.

F. betulina grows exclusively on birch, and the snag it is growing on appears to be birch based on the light color and horizontal lenticels. Birches do have similar leaves to beeches, though the surrounding trees could be different species.

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    $\begingroup$ Thanks for the guess! Looking at photos of F. betulina, the top and the bottom looks different (the top is brownish and round, the bottom is flat). This one was more or less symmetric. I don't recall seeing any which looked asymmetric, but then it was two years ago ... Yes, this is a lone birch stump and the surrounding trees are beeches. $\endgroup$
    – Szabolcs
    Sep 6, 2022 at 7:58
  • $\begingroup$ @Szabolcs The mature mushrooms I linked earlier don't really look like what you found, but see the image I just added - when they're young, they don't have the pore surface on the bottom. $\endgroup$
    – Eonema
    Sep 6, 2022 at 16:28
  • $\begingroup$ Looks convincing enough! $\endgroup$
    – Szabolcs
    Sep 6, 2022 at 16:51

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