Euphorbia / Cactus Rotting From Base – Need help saving it

Hi everyone,

I’m trying to understand what happened to my euphorbia-type cactus and whether my current actions are and were correct. I’m not an expert but usually my plants do fine…

It started, I believe, because I watered it but where I live it became very cold and my house is also very cold at night. I notice one morning the cactus was brown at many spots … So after googling and ChatGPT etc, it seemed it caught a cold. I moved it to a warm room (20 Celsius) with lots of light and ventilation. Here is what’s up now:

Context / Environment

• Indoor plant, next to large windows

• Very bright light, lots of sun during the day

• Room temperature stable around 20–22°C day and night

• Not cold at all, even at night

• Watered roughly once a month

• Soil always looked dry on the surface

Initial Symptoms

• Lower parts started turning brown

• Thought it might be corking or cosmetic

• Plant still firm higher up

• Continued to get new brown spots daily

Progression

• Base became softer and slightly mushy

• Brown area spreading upward slowly

• Some branches looked dry outside but inside were still green

• When cutting some parts, some “bled” white sap, others were dry

• Soil looked dry but rot continued anyway

What I Suspect Happened

• Root rot or internal stem rot started earlier (probably from old overwatering or poor drainage)

• Infection moving upward internally even though outside looked dry

• Not a temperature issue, more likely drainage / internal rot

Steps I Took

• Removed all clearly rotten / yellow / mushy arms

• Clean cuts with knife

• Checked cross-sections: some green and dry inside, some brown

• Kept room warm and bright

• Stopped watering

• Soil is dry now

• But base still slowly getting softer and browner

Current Situation

• Top of plant still looks green and firm

• Base continues to soften slightly

• Concerned rot is still progressing internally

Plan / What I’m Considering

• Cutting the entire top above any softness

• Letting it callus for \~7–10 days

• Re-rooting in fresh cactus soil with proper drainage

Main Questions

• Does this sound like classic basal rot?

• Is cutting the top and re-rooting the only real solution at this stage?

• Anything else I should check before doing the “amputation”?

Thanks in advance, I’m really trying to save at least the healthy top if the base is doomed.