Activated charcoal is often used in plant tissue culture systems to reduce phenolic oxidation and improve cell growth and development. This study investigated activated charcoal's effect on the somatic embryogenesis of dragon fruit (Hylocereus species). Nine-month-old seed-derived dragon fruit stems wereplaced in basal Murashige and Skoog (MS) plates supplemented with 2, 4-D (1.5 mg/l) and activated charcoal (1g/l). Cultures were incubated in 14 h continuous light or under dark condition. MS medium without activated charcoal served as the control. Percent fungal contamination, degree of tissue browning, and callus growth (length, width, and total area) were assessed. A significant decrease in contamination was observed four days after inoculation (dai) in a medium containing activated charcoal regardless of the light condition used (p<0.05, T-test analysis). In contrast, no significant difference in fungal contamination between the two treatments was found eight days after incubation (dai). Fungal contaminants’ identities were done using a combined morphocultural and molecular analyses. The fungal isolates showed thick and white to grayish mycelia without spore production seven dai in potato dextrose agar medium. DNA sequence analysis showed a high percent similarity of the isolates to two Diaporthe species. Activated charcoal did not affect the length, width, and total area of the calli produced. However, a significant decrease in tissue browning was recorded (p<0.05). Overall results demonstrated the applicability of activated charcoal in dragon fruit tissue culture to reduce in-vitrobrowning. Additionally, examining further the possible role of fungal contaminants in dragon fruits would be valuable. A hedonic scale that can be used to assess tissue browning in dragon fruit is also described.

Dragon fruit (Selenicereus spp.) is a cash fruit crop worldwide. One of the most effective techniques in producing clean planting materials and preventing disease infection is a plant tissue culture system. This study investigated the identity and characteristics of fungal contaminants in tissue‐cultured red‐fleshed dragon fruit (S. monacanthus) stem explants. Four fungal isolates were recovered from contaminated S. monacanthus cultures. The fungal isolates showed thick and white to greyish mycelia 7 days after incubation (dai) in potato dextrose agar medium with spore production 13 dai. The isolates were identified as Diaporthe sp., based on combined morphological and cultural characteristics. DNA analysis of the partial sequence of the ribosomal internal transcribed spacer (ITS), beta‐tubulin 2 (TUB) and transcription elongation factor 1‐α (TEF1‐α) gene regions revealed a high similarity of the isolates to D. passifloricola, D. limonicola and D. tectonae. In detached stem and glasshouse assays, the three Diaporthe species identified were found pathogenic to three dragon fruit species (S. megalanthus, S. undatus and S. monacanthus). Early symptoms observed were similar to the disease of dragon fruit (caused by the same fungal genus) previously reported in Malaysia. The fungi were re‐isolated from the inoculated stems, establishing Koch's postulate. This study confirms three Diaporthe species as contaminants in dragon fruit explants, reports the susceptibility of S. monacanthus and S. undatus to a stem disease, and the tolerance of S. megalanthus to the disease caused by Diaporthe species.