Sudden Decline in Euphorbia erythraea: Basal Rot Diagnosis

A 1-year Euphorbia erythraea in warm room, watered when soil "rock hard," suddenly shows brown base with white fluffy mold. Rot likely started in roots weeks ago. Unpot: if roots white/firm and brown part hard, spray fungicide, repot in sterile soil/new pot. If roots bad/tissue soft: cut 3–4" above rot to healthy tissue, sterilize knife between cuts, callous 2 weeks, root in well-drained sterile mix, plant deep in barely moist soil, mist only. Tug gently in 2 weeks—resistance = roots. Water gradually. Euphorbias dislike bone-dry soil; water when top inch dry. Mist every few days in warm dry air. Balance: not too wet, not too dry.