Why is my plant turning brown?
On succulents especially, browning is usually the visible end of a root problem that started weeks earlier.
The causes, and how to tell them apart
Root rot from overwatering most common on succulents
How to tell: Browning that starts at the BASE and moves upward, with soft or mushy tissue near the soil line. The plant may feel loose in its pot.
What to do: Unpot immediately. Cut away all soft brown tissue, let the cut surfaces callus for a day or two in open air, then repot into dry gritty mix and wait a week before watering.
Sun scorch common after a move outdoors
How to tell: Bleached, tan or reddish-brown patches on the surfaces most exposed to sun, appearing within days of a move into brighter light.
What to do: Move to morning sun only and reintroduce brighter light gradually over two to three weeks. Scorched tissue will not recover.
Cold damage seasonal
How to tell: Browning after a cold night, often mushy at first and then dry and papery.
What to do: Bring it in ahead of cold nights. Remove the damaged growth once it is fully dry.
By species
The likely answer differs by plant — these species have their own version of this problem:
Plantroster compares this month's photo against last month's, so slow changes — the ones you stop noticing because you see the plant every day — get flagged.
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General horticultural guidance, not a diagnosis of your specific plant.