Black Spots on Small Cacti: Root Mealybugs or Rot?

Colby's year-old trio of skinny, soft-spined cacti developed tiny black spots near the top of one plant, appearing to eat into the flesh.

Investigation Steps

1. Unpot and check roots: healthy roots are off-white; brown/black indicates rot.
2. Look for root mealybugs: tiny white cottony masses on roots.
3. Ensure porous soil and warm temps (above 50°F).

Likely Causes

• Overwatering in heavy soil → root rot → black spots on stems.
• Root mealybugs feeding → plant decline → secondary fungal spots.
• Pot-bound roots → poor water uptake → tissue death.

Action

• Repot in fresh cactus mix with added pumice.
• Treat mealybugs with systemic insecticide or alcohol-dipped swabs.
• Water only when soil is completely dry.

Early intervention can save the collection.