White Mold on Cactus Soil: Repotting and Drainage Fixes

A repotted cactus declined with white fungal growth on the soil surface. The owner suspected the fungus was killing the plant.

Diagnosis

The fungus is likely saprophytic—feeding on decomposing organic matter in the potting mix, not attacking the cactus. The real problem is almost certainly poor drainage and/or overwatering, which also favors fungal growth.

Corrective Action

1. Bare-root the cactus and inspect roots. Trim any black, mushy sections.
2. Repot in coarse cactus mix (1:1:1 potting soil, coarse sand, perlite/pumice).
3. Clean pot with 10% bleach solution before reuse.
4. Withhold water 1 week after repotting to let root wounds callous.
5. Water sparingly thereafter—only when soil is dry 2 inches deep.

If Fungus Returns

Apply a labeled succulent/cactus fungicide per instructions. Avoid home remedies; they can damage sensitive tissues.

Cacti demand sharp drainage and infrequent watering. Fix the environment, and the fungus disappears.