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I had a plant with a fungus growing in the soil. I have since replanted the plant with new soil and in a new pot. Since the old pot was rather big, I am wondering if I could reuse some of the soil in it? It appeared the fungus was just on the top and I have thrown out the top half of the soil in the pot. Can I reuse the soil at the bottom or can it have been infected with the fungus? Should I clean the pot with soap or is fungus not that "contagious"?

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  • what kind of fungus? white fluffy stuff on the surface of the soil, slime mould, mushrooms, toadstools?
    – Bamboo
    Jun 25, 2021 at 13:24
  • @Bamboo one of it was white fluffy, but then there was one part that stood up erect and was yellow. See picture here gardening.stackexchange.com/questions/57944/… Jun 26, 2021 at 11:45

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Thanks for the link. You can reuse the soil, but only if you don't mind more fungal growth - the soil obviously contains mycelium, which you cannot see, and mycelium produces fruiting bodies such as toadstools, mushrooms and the extrusion present in the linked picture, depending on which type of mycelium is present. If you have a garden, put the soil out there, rather than reusing it in a pot indoors. As for the pot, if you want to plant something else in it, then cleaning thoroughly is always a good idea before reuse.

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It doesn't look like a fungus or mushroom to me. I think you have slime mold, something in the Arcyria genus. Mycelium doesn't usually grow above the top inch of soil--but slime mold colonies will come up if conditions are right.

They're really cool and most species are benign to plants. They're not a mold, actually related to amoebas! They live as single-celled organisms separate from each other but when the environment is right (wet conditions on the soil surface, lots of them in one place) they come together and merge into a blob to form the reproductive bodies and send out spores. The bright yellow is the reproductive spore dispersal mechanism and the light yellow/whiteish areas are smaller slime mold blobs. Those blobs can move by gliding on the surface of soil! They're slow, but cool to see.

Anyway, as for the original question, yes, the pot and soil is contaminated. Individual slime molds are single celled organisms, and they can reproduce by meiosis (one cell splitting into two). You can clean the pot, leave it out in the sun to dry and you should be good...but it came from somewhere and that somewhere might be your soil itself!

They're not going to hurt the plant unless it's a small seeding. They're part of the natural decomposition process, just like mushrooms (and fungus and mold, though these latter two are more likely to be harmful to plants). You can manage it by keeping the top layer of soil dry and limiting organics (leaves, wood, peat, etc) at the top. Mulch or gravel will help, cutting down on watering will help more--water less often but more deeply. Pick them out as you see them come to the top and eventually the population will stabilize where it doesn't form these blob colonies.

I could be wrong about this being slime mold, but the same recommendation concerning water applies even if it was a fungus.

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