Definitive Strategy: Bring Inside, Cut Rot, Re-root

Conflicting online advice creates confusion. Here's the proven protocol:

Step-by-Step

1. Move indoors immediately—cold + wet = death for non-hardy cacti.
2. Unpot, shake off soil gently.
3. Sterilize knife; cut rot back to healthy tissue.
4. Clean knife after every cut (alcohol/flame).
5. Check vascular bundles for orange-brown streaks—cut further if found.
6. Destroy infected material and soil (don't compost).
7. Dust wounds with fungicide, rooting powder, or charcoal.
8. Air-dry plants briefly.
9. Clean/disinfect pots or use new.
10. Repot in fast-draining mix: ⅓–½ peat/coir, equal parts sifted builder's sand and perlite. Optional: blood/bone meal, charcoal, dry fungicide.
11. Moisten mix until it crumbles when squeezed.
12. Don't water 1 week; stake if wobbly.
13. Water infrequently; mist weekly.
14. Cool, sunny indoor location until re-established.

Winter Watering Adjustment

Normally: zero water Oct–March, cool room, occasional mist. For recovering plants: slightly more generous but still very sparing.

Insurance

Cut and root a few offsets separately as backup.

Rooting Note

Cactus rooting is a separate skill—patience required (months). Your overnight-shriveling cutting was anomalous; proper callousing prevents this.

Identify the species if possible for tailored care. Good luck—you can save these prize specimens.