Rhizome or crown rot

What's happening

When the fleshy rhizome stays too wet — from overwatering, water pooling in the crown, or poor drainage — it rots from the inside even though the surrounding soil looks merely damp. With its base failing, the plant can no longer move water up, so every leaf wilts at once regardless of how moist the mix is.

How to fix it

Unpot the plant right away and examine the rhizome. Cut away any brown, soft, or foul-smelling sections with sterilized snips until only firm, pale tissue remains. Repot the healthy portion shallowly into fresh, fast-draining mix in a clean pot with drainage, keeping the rhizome resting on the surface rather than buried. As insurance, take a few healthy leaf or rhizome cuttings and propagate them — rex begonias root readily and give you a backup if the parent can't recover.

This is general guidance based on common symptoms; individual plants vary.