I have the same problem with a houseplant that I think is a begonia or some such variety. My sister informed me last week that the potting mix is sour, so time for a repotting session I think. Many of my plants are looking neglected so that a job for this week whilst I'm on leave.
I have a unproven theory about these toadstools too, that they perhaps appear when the humidity is up, and probably because my watering isn't when needed but a soaking when poor plants are desperate!!!!! Plus of course the sour potting mix.
= NEGLET!!!!!!!!!!
Please don't report me to the GREENIES!!!!!!
Regards,
Raewyn G
:-o