The soil stays wet for days, and the rot starts low: a soft, discolored patch at the base rather than one particular old leaf.
Small dark flies around moist potting medium may be fungus gnats.
The fix
1
Stop watering completely.
2
Wait until the top 5 cm (2 in) of soil feel dry before you even think about the next one, and check the pot actually drains.
3
If only some roots are brown and mushy, trim them and repot in fresh, fast-draining mix.
More about this
They suggest conditions suitable for breeding, without proving that the surface never dries.
If the base itself is soft, the damage is more serious and recovery is uncertain. Only propagate using a method suited to this plant, not arbitrary pieces above the rot.
Other possible causes
Not a match? Compare the signs below to find the one you have.
Too little light
The yellowing comes with pale, stretched new growth leaning toward the nearest window.
The fix
1
Move it to a brighter spot, but keep it out of harsh midday sun, which scorches.
More about this
That combination shows that the spot is too dim for a zebra haworthia.
Thirst (underwatering)
The soil is bone dry, and the yellowing arrives alongside wrinkled leaves rather than soft, mushy growth.
The fix
1
If the soil is dry, water thoroughly (every 11 to 17 days in summer is the usual pace for a zebra haworthia), then settle into feeling the soil before every watering rather than watering on a fixed day.
Hungry (but rarely the real culprit)
Zebra haworthia grows slowly and needs very little feeding, so a uniform pale-yellow fade after many months without any is possible, but check watering and light first: those are far more common on this plant.
The fix
1
Use a cactus and succulent fertilizer at half the strength on the label, about every 180 days while it’s actively growing.
2
Overfeeding a slow grower like this does more harm than skipping it.
Simple old age
An older, lower leaf slowly yellows or shrivels on its own while new growth looks plump and healthy.
How to tell
That’s normal turnover on a zebra haworthia, not a symptom.
The fix
1
Nothing is wrong.
2
Remove the spent leaf once it comes away easily and carry on.
