Alternaria

Alternaria is a family of fungal pathogens affecting Trichocereus species. The infection physically manifests as distinct lesions that alternate light and dark colored on the epidermis that can eventually crack as the disease progresses. It causes severe, permanent scarring on the skin of the cactus.

Fungi of the genus Alternaria have been widely documented as a persistent, hard-to-eradicate pathogen. Breaks in the plants outer skin can allow fungus to get a foothold. Cacti become more susceptible to fungal infections when deprived of certain essential nutrients, such as calcium and silicon. Wet, humid conditions, excessively frequent watering, or plants sitting wet for extended periods of time can allow fungus to survive on cacti skin until they manage to penetrate and begin an infection.

• Environmental Sensitivity: The pathogen spreads rapidly in damp garden conditions and is highly active during wet or rainy seasons.
• Watering: Soil splashing up onto the column while watering is an infection vector. Minimize splashing during watering.
• Surgical Management: If the infection is localized, growers recommend cutting the cactus to salvage the healthy tissue. The infected portion should be discarded immediately to prevent the fungus from spreading to neighboring plants.
• ** Repotting damage:** If the skin of the lower column near the soil line is broken during repotting or a column is buried too deeply, it can allow an infection to take hold. Watch for column damage and if spotted set aside to callous before repotting. Avoid raising the soil line significantly.
• Root Vulnerability: If the infection establishes below the soil line near the root system, it is typically fatal; the plant may rot internally and die before showing outward signs of collapse.
• Chemical Treatment: The systemic fungicide propiconazole has been proven by community members to be an effective treatment, though it is not safe for food crops. The surface contact fungicide Mancozeb is effective to apply topically to halt spread.

Analysis of the discussion shows that growers frequently struggle with identifying the pathogen, often confusing it with physical damage or tumors before recognizing its fungal nature. The community consensus emphasizes physical isolation and surgical intervention over chemical treatments, which remain largely untested among hobbyists.

Frits Potgieter was assigned a high confidence score (7) due to providing detailed, practical observations regarding the pathogen's behavior during rainy seasons, its impact on root systems, and specific grafting management strategies. Peter Maxwell was rated 6 for providing the initial identification of the pathogen, which aligns with the physical symptoms described by other growers. Gee Bee did not participate in this discussion.