Bipolaris (Bipolaris cactivora tip rot)

Bipolaris tip rot is a highly destructive fungal disease caused by the pathogen Bipolaris cactivora that primarily targets the growing tips (apices) of Trichocereus cacti.

The disease manifests physically at the apex of the plant:

• Initial Stage: It presents as a wet, “black goopy mess” (often referred to as “weeping black rot”). The epidermis color of the affected area transitions from healthy green or glaucous blue to a deep purple, and eventually to a dark, necrotic black.
• Progression: As the infection runs its course or dries out, the affected tissue turns grey, crusts over, and begins to sporulate.
• Growth Arrest: The rib structure at the tip is typically destroyed or severely distorted, leading to arrested growth. If the plant survives, it walls off the infection, resulting in a scarred, grey apex from which new ribs and growth tips (offsets) eventually emerge.

Bipolaris tip rot is caused by the fungal pathogen Bipolaris cactivora. Outbreaks are heavily associated with seasonal transitions, high rainfall, and sudden cold snaps. It is often confused with other top-down fungal pathogens like Fusarium proliferatum and Neocosmospora, or bacterial wet rot (such as Erwinia carotovora).

• Environmental Management: Avoid excessive watering during cold snaps and seasonal transitions. Protect plants from freezing temperatures, which cause cells to expand and burst, creating entry points for the fungus. Raising the soil line or mounding can help prevent wet roots.
• Soil & Nutrition: Ensure proper soil pH and nutrient balance. Hindered calcium availability (often caused by pH imbalances or excess of other nutrients) can weaken the plant's defenses. Some growers apply dolomite lime (raises soil pH) or gypsum and epsom salts as part of a winter routine.
• Chemical Treatment:
• Phosphorous Acid (Phos Acid): Highly recommended as a preventative and active treatment. For active infections, spray directly on the wound. Brad Albert suggests using up to three times the recommended dose for virulent cases, applying repeatedly until the infection dries, crusts, and turns grey.
• Propiconazole: Recommended by community authority Gee Bee, though she notes that true Bipolaris may not respond to it, requiring physical removal. Note: Propiconazole is not recommended for edible plants (labeled for turf and ornamentals).
• Sulfur Powder: Generally ineffective at stopping systemic or active outbreaks, as the rot may reappear elsewhere.
• Physical Intervention (Chopping): If chemical treatments fail or the rot continues to spread down the ribs, cleanly chop the infected tip. Deciding to chop immediately versus “letting it ride” is a common dilemma; some plants successfully throw off the infection and offset naturally, while others will rot entirely if not amputated early.