Neocystalidium dimidiatum

*Neocystalidium dimidiatum* is a fungal pathogen known to affect cacti, including *Trichocereus* species. Rather than a clone, it is an infectious agent that presents a significant challenge to growers.

The pathogen primarily attacks the epidermis of the host plant, producing visible lesions and scarred markings. These symptoms can compromise both the long-term health and the aesthetic value of the cactus. Because the fungal lesions often present as silvery, winding scars across the ribs and areoles, they are frequently misidentified by growers as superficial pest damage.

The impact of this pathogen within the *Trichocereus* growing community was notably documented around January 2026, when grower Dylan Gordon shared images of an affected specimen. While initial community speculation attributed the epidermal scarring to pests, a definitive diagnosis of *Neocystalidium dimidiatum* was provided by community authority Gee Bee, establishing a baseline for identifying and treating the disease within private collections.

Managing a *Neocystalidium dimidiatum* infection requires a combination of chemical intervention and cultural adjustments: * Chemical Treatment: Systemic fungicides are necessary to halt the spread of the pathogen. Highly effective options include strobilurins and dimethyl inhibitors (DMIs)—specifically, applications of azoxystrobin and difenconazole. * Substrate & Moisture Control: Fungal pathogens thrive in stagnant, humid conditions. It is highly recommended to transition affected plants into a highly porous, fast-draining soil mix to minimize moisture retention around the root zone. * Ineffective Treatments: Growers should avoid applying snail bait or molluscicides. Although the scarring superficially resembles snail trails, pest control measures will not resolve this fungal infection.