A week ago I watered my orchid and forgot it was sitting in the water tray. Now it has a weird smell and when I opened the ceramic pot I saw some black spots on the roots. How do I know if it's a fungus and what can I do to save it?
The first thing is to get the orchid out of the water and let it dry completely, because fungi feed on moisture. Those black spots on the roots are almost definitely fungus, especially if it smells rotten or like wet soil; in that case you need to carefully remove all the black or soft root (healthy roots are white or grayish) and repot it in fresh, dry substrate, preferably orchid bark. From now on, only water it when the substrate is completely dry, which is normally every 7-10 days depending on where you live, and never leave it sitting in trays with standing water because that's what kills these plants.
Those black spots are fungus, that's clear, but before you just let it dry out you need to check the roots carefully and cut off any that are soft or completely black with a clean tool (alcohol or fire) because if you leave them there they're going to keep rotting. The thing nobody mentions is that ceramic pots absorb moisture and it's hard to dry them out inside, so if you actually want to save the orchid, consider switching it to a transparent or plastic pot where you can see what's happening with the roots and it dries faster. Once you cut out the rotted parts, repot it with fresh bark (never reuse the old stuff) and don't water it until you see it's almost dry.
If the root is already soft or completely black, that part is dead and you need to cut it off without hesitation. Use scissors or a knife disinfected with alcohol or diluted bleach, and don't be afraid to be aggressive: it's better to lose a piece of healthy root than to let the fungus spread.
After cutting, what really works is changing all the substrate. People tend to think that just letting it dry is enough, but the moss or bark where the pot was living is still full of spores. Take the orchid out, get rid of all the old soil, dry the roots well with paper towels (without rubbing), and repot it in fresh, clean substrate. While you're doing this, check that there are no black spots left on the roots you want to keep; if you're in doubt, it's better to cut.
Once planted, don't water for a few days. After that, water but be really careful with standing water: if you use a pot with drainage holes and put it on a tray, take the orchid off it after a few minutes. The fungus needs that humid, waterlogged environment to thrive, so the best thing is not to leave it on the water tray again. The odd smell should disappear in a couple of weeks if the plant starts to recover.
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