Weeping Black Rot [San Pedro Cactus Wiki]

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====== Weeping Black Rot ======
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===== Overview =====
Weeping Black Rot is a community-suspected environmental disorder or disease affecting cacti, particularly when grown in hot, humid climates. It is characterized by dark, weeping lesions that typically manifest when plants are exposed to stagnant air, high temperatures, and excessive moisture. This condition is of significant concern to greenhouse growers in humid regions, as it can quickly damage tall, established specimens.

===== Symptoms & Identification =====
The condition often begins with a deceptive, "brilliant purple blue" discoloration on the epidermis of the cactus, which growers note can look attractive before progressing. As the disease advances, it develops into dark, weeping rot lesions. It commonly manifests on the upper sections of tall cacti (often over 7 feet high) that have grown into the hotter, more humid upper zones of greenhouses. It is also associated with stress following winter temperature extremes, such as nights dropping into the low teens. 

===== Cause & Spread =====
The precise pathogen causing Weeping Black Rot is unconfirmed in the community discussion. However, its development is heavily linked to environmental and cultural factors:
* **High Humidity and Heat:** Stagnant, hot air in the upper levels of greenhouses is a primary trigger. Dion Gryphon (reputation 6. noted that the condition "can be caused by humidity."
* **Soil Acidity and Mineral Deficiency:** Reporter Jason Mcghee noted the rot coincided with plant roots reaching acidic native clay beneath prepared soil beds, suggesting a lack of calcium may contribute to susceptibility.
* **Regional Climate:** It is highly prevalent in humid regions of the southeastern United States, including North Carolina, Texas, and Florida. Phil Mack (reputation 6. observed: "Unless youve got a super well tuned setup your cactus in states like florida and other SE U.S. states are going to be effected by stuff like this to some extent."

===== Treatment & Management =====
Management focuses on environmental modification and physical removal of affected tissue rather than chemical treatments:
* **Airflow and Ventilation:** Improving greenhouse ventilation is critical. Peter Nehen (reputation 6. and other growers emphasize that airflow is a major factor in humid climates like Central and Northeast Texas. Growers recommend opening up the top ends of greenhouses and adding multiple fans to keep air moving and lower temperatures in the upper canopy.
* **Soil Amendments:** Adding lime to the soil can help neutralize acidity and supply calcium if roots have penetrated acidic native clay.
* **Pruning:** Affected areas should be cut away ("a chop not a rescue mission") to prevent the rot from spreading down the column.
* **Sunlight Exposure:** Ensuring plants receive direct sunlight helps dry out the substrate quickly after watering or rain. Phil Mack noted that placing cacti in direct sun allows the substrate to dry much faster than keeping them under shelter where they might still get partially wet but lack the sun to dry out.

*Note: Gee Bee did not participate in this discussion, and no official treatment protocol from them is available.*

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===== Reported Cases =====
//Visible to trusted reviewers only. The photos above belong to these reporters; credit them.//

^ Date ^ Reporter ^ Diagnosed by ^ Key note / quote ^
| 2026-06-22 | Jason Mcghee | Dion Gryphon | "To bad the brilliant purple ble is disease because at the beginning it looks really cool." |

===== Contributor Confidence =====

^ Contributor ^ Confidence ^
| Dion Gryphon | 6.7/10 |
| Robby Lapp | 6.6/10 |
| Peter Nehen | 6.5/10 |
| Phil Mack | 6.0/10 |
</ifauth>

===== Notes & Contradictions =====
Gee Bee did not participate in this thread. Consequently, there are no authoritative statements, evolving opinions, or contradictions from Gee Bee to document. The community consensus, led by Dion Gryphon and Jason Mcghee, attributes the condition to environmental stress (humidity, heat, and soil acidity) rather than a specific infectious pathogen, though the exact biological cause remains unconfirmed.

===== References =====
* **Dion Gryphon** (Disease identification and humidity link)
* **Jason Mcghee** (Case reporter, photo source, and observations on greenhouse heat, soil acidity, and lime treatment)
* **Peter Nehen** (Observations on airflow in humid climates)
* **Phil Mack** (Observations on climate challenges in the southeastern U.S. and substrate drying)
* **Robby Lapp** (Inquiries regarding environmental conditions)