Bipolaris (Bipolaris cactivora tip rot)

Bipolaris tip rot is a destructive fungal disease caused by the pathogen Bipolaris cactivora that primarily attacks the growing tips of cacti. It is characterized by rapid tissue breakdown at the apex, often triggered by environmental stress such as cold snaps and high moisture. If left unchecked, it can cause severe tip dieback, though plants may survive by offsetting or growing through the damage once the infection dries.

The disease typically manifests at the apex of the plant. * Initial Stage: It appears as a wet, “black goopy mess” at the growth tip (often referred to locally as “weeping black rot”). * Progression: As the infection runs its course or dries out, the affected tissue turns grey, crusts over, and begins to sporulate. * Growth Arrest: The cactus tip will stop growing. In some cases, the plant successfully walls off the infection, and a new tip eventually pushes through or offsets. * Lookalikes: Community authority Gee Bee notes that Bipolaris is easily confused with other top-down fungal pathogens, specifically Fusarium proliferatum and Neocosmospora. It is also sometimes misidentified as bacterial wet rot (such as Erwinia carotovora).

The primary pathogen is the fungus Bipolaris cactivora. * Environmental Triggers: Outbreaks are heavily associated with seasonal transitions, high rainfall, cold snaps, and wet roots. Freezing temperatures can cause cells to expand and burst, creating entry points for the fungus. * Soil & Nutrition: Loo'k Lost-mang suggests that widespread outbreaks may be linked to hindered calcium availability, potentially caused by soil pH imbalances or an excess of other nutrients. * Vectors: Mealybugs and other pests are frequently found near or inside the wounds. While Gee Bee notes that insects (such as ladybugs and mealybugs) may simply be attracted to the site to feed on the fungal hyphae and spores (acting as an early warning sign), growers like Wilfy Fletcher suspect pests may also vector or introduce the pathogen to greenhouse environments.

* Chemical Control (Propiconazole): Topic authority Gee Bee recommends trying propiconazole. However, Gee Bee notes that if the pathogen is indeed Bipolaris, it may not respond to this treatment, necessitating physical removal. * Chemical Control (Phosphorous Acid): Brad Albert recommends spraying the infection directly with phosphorous acid (phos acid) to dry it out, noting success using up to three times the recommended dose for virulent cases until the tissue crusts and turns grey. Wilfy Fletcher and Matt Gamble also support phos acid as a garden staple for this issue. * Physical Intervention (Chop): If chemical treatments fail or the rot continues to spread down the ribs, the infected tip must be cleanly cut off (chopped) to save the rest of the plant. * Cultural Controls: Raising the soil line, mounding, protecting plants from winter wetness/cold snaps, and monitoring soil pH can prevent outbreaks.

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

* Fungal vs. Bacterial: Wilfy Fletcher initially suggested Bipolaris was bacterial, but Loo'k Lost-mang corrected this, clarifying that Bipolaris cactivora is fungal, whereas Erwinia carotovora is the bacterial equivalent. * Treatment Discrepancy: While Brad Albert advocates for heavy applications of phosphorous acid to halt and dry out the rot, Gee Bee notes that true Bipolaris may not respond to systemic fungicides like propiconazole, making physical amputation (“chopping”) the definitive cure if it spreads.

* Topic Authority: Gee Bee * Contributors: Brad Albert, Loo'k Lost-mang, Wilfy Fletcher, Thomas Mitchell, Chad Rhodes, Peter Maxwell, Hayley Marie, Phil Mack, Jake Blackshear * Photo Credits: Dom PassdaMyc, Wilfy Fletcher, Thomas Mitchell, Nicholas Lindsey