Bipolaris

Bipolaris is a fungal disease affecting cacti that primarily targets the growing tips of the plants. It is characterized by a wet, dark rot that typically emerges during periods of environmental stress, such as cold snaps combined with high moisture. While it can look alarming and cause tip dieback, infected cacti often survive and can grow through the damage with proper treatment or environmental adjustment.

The primary identifier of Bipolaris is a “black goopy mess” that typically appears at the growth tips of the cactus, a symptom also referred to by some growers as “weeping black rot.” As the infection progresses and dries out, the affected tissue turns grey and crusty. It is at this dry, grey stage that the pathogen releases its spores. Bipolaris is sometimes confused with bacterial wet rots, such as those caused by *Erwinia carotovora*.

The disease is caused by the fungal pathogen *Bipolaris cactivora*. Its development is heavily driven by environmental factors, particularly seasonal transitions, excessive rainfall, and cold snaps that cause cell damage in the tips or roots. Additionally, soil chemistry may play a role; Loo'k Lost-mang suggests that widespread outbreaks may be linked to hindered calcium availability in the plant, often caused by soil pH imbalances or an excess of other competing nutrients.

*Note: Gee Bee did not participate in this community discussion. The following protocols represent the consensus of experienced growers in the thread.*

* Phosphorous Acid (Phos Acid): This is the primary treatment recommended by the community. Brad Albert advises spraying the infection directly. For virulent cases, he recommends using approximately three times the standard recommended dose, applying it repeatedly until the wet rot visibly dries, crusts over, and turns grey. * Preventative Spraying: Applying phosphorous acid as a preventative spray during winter transitions can significantly reduce the occurrence of seasonal outbreaks.

* Soil and Nutrient Management: Checking soil pH and ensuring adequate calcium uptake can help prevent the physiological weaknesses that allow the fungus to take hold. * Physical Intervention: If the rot is severe or occurring during cold, wet weather, the infected tip can be cut off. Hayley Marie notes that trimming should ideally be done on a warmer day, and the recovering plant may need temporary shelter. * Observation: Some growers, including Peter Maxwell and Chad Rhodes, report that healthy plants frequently “throw off” the infection without active treatment, arresting their tip growth temporarily before pushing out a clean new tip.

Community examples - the photos above belong to these reporters; credit them.

* Pathogen Classification: Wilfy Fletcher initially suggested that Bipolaris is a bacterial issue. Loo'k Lost-mang corrected this, clarifying that *Bipolaris cactivora* is a fungal pathogen, whereas bacterial rot is typically associated with *Erwinia carotovora*. Brad Albert confirmed its fungal nature by noting that the dry, grey phase of the lesion is when the pathogen sporulates. * Application Strength: Thomas Mitchell noted he had previously experienced poor results using weaker dilutions of phosphorous acid. Brad Albert clarified that standard doses are often insufficient for active, virulent outbreaks, and that he successfully “nukes” the infection using three times the recommended label dose.

* Topic Authority: Gee Bee did not participate in this discussion. * Contributors: Brad Albert, Loo'k Lost-mang, Chad Rhodes, Peter Maxwell, Hayley Marie, Thomas Mitchell, Wilfy Fletcher, Jake Blackshear, Matt Gamble, Dion Gryphon. * Photo Credits: Thomas Mitchell, Nicholas Lindsey.