Identification of three Diaporthe species from Selenicereus sp. explants and their pathogenicity to three dragon fruit species

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.