Root Rot Fungal Infection on opuntia
What's Happening
Chronic soil saturation from overwatering combined with moisture-retentive organic soil creates anaerobic conditions promoting fungal pathogens. Alternaria alternata, Colletotrichum gloeosporioides, and Aspergillus tubingensis are primary pathogens causing black necrotic spots and basal rot in Opuntia species. The cladode tissue's high water content makes cacti vulnerable to rapid fungal spread once infection establishes at the soil line or wound sites.
How to Fix It
- 1
Inspect and excise all rotted tissue with sterile knife, cutting 1-2 cm above visible rot until only firm green tissue remains
- 2
Allow cut ends to callus completely in dry, shaded area with good airflow for 7-14 days until hard and dry
- 3
Repot in sterile, gritty mix: 70-80% inorganic material (perlite, pumice, coarse sand) with 20-30% cactus soil
- 4
Plant shallowly with callused end just touching soil surface; provide support stakes if needed
- 5
Withhold water for 2-3 weeks post-planting, then resume minimal watering only when soil is bone-dry 2-3 inches deep
How to Prevent It
Water only when soil is completely dry throughout the pot; use fast-draining inorganic substrate (minimum 70% drainage material); ensure pots have drainage holes; maintain low humidity (<50%) and excellent airflow; avoid organic mulches or moisture-retentive top dressings; sterilize tools between plants to prevent cross-contamination.