"Newt" ha scritto nel messaggio
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Mushrooms come from spores. The spores were probably in the soil or
the mulch you used. They probably didn't come with the seeds. Then
again, these are WILD strawberries, so there may have been mushroom
spores. Just pluck the 'shrooms out.

Thanks for your answer.
Even if I begin guessing that those seeds are guilty, because the soil is
new and I sowed at home (indoor = it's not subjected to the wind and what
it may carry)
Anyway I'll wait to know if something else sprouts...