Stem and root rot

What's happening

Yucca stores water in a thick woody trunk, and when the roots sit waterlogged that rot creeps upward into the cane. Affected areas go dark, soft, and spongy — you can often press a thumb into them — and may give off a sour smell. Left unchecked, the rot hollows out the trunk and the whole plant collapses.

How to fix it

Act quickly. Unpot the plant, rinse the roots, and cut away every soft, brown, mushy root and section of trunk with sterilized blades until you reach firm, pale, clean tissue. Let the cut surfaces callus over for a day or two in the open air, then repot the healthy portion into fresh, gritty, fast-draining mix in a clean pot with drainage holes. Water very sparingly while it re-roots, and place it somewhere warm and bright to recover.

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