Toadstools!

Hi all,

I have 6 plants growing in one location, but about 4 days ago one of them inexplicably started drooping, and now appears to have died. Today I noticed that there are a number of thin white toadstools growing in the pot.

I'm not sure which is the chicken and which the egg here: I watered the plants including fertiliser the day before, and may have included a bit too much of one/both. I also slightly moved the daylight hours back for better temps and reduced the amount of ventilation (which was excessively high). The day after that is when I noticed that plant was dying.

So I don't know what the cause is: it received the exact same treatment as the other plants which are all ok. The way it just died overnight suggests to me it caught a serious fungal infection, but it could also be that it just couldn't handle the changes (maybe it got the last uberconcentrated bit of the watering can?), and the fungus started growing around it after (because) it had died.

Obviously I'm a bit worried about my other plants as I don't want them to keel over, especially since a couple of them have just started to go nicely into budding.

Any ideas anybody.