First Report of Stem Rot in Dragon Fruit ( <i>Hylocereus polyrhizus</i> ) Induced by <i>Diaporthe tulliensis</i> in Guangxi Province, China

Guangxi is the dominant production region for dragon fruit (Hylocereus polyrhizus) in China, supplying ~40% of the total annual harvest. In July 2024, stem rot was observed in dragon fruit plants from a Long'an farm (23°0′39″N, 107°52′17″E), with a ~20% incidence. Affected stems exhibited soft decay with yellowing and localized reddening. To isolate the pathogen, nine representative plants were collected from three symptomatic plots. Stem segments 5×5 mm were surface-sterilized using 1% (v/v) NaClO for 2 min. Subsequently, segments were triple-rinsed with sterile water, and the epidermis was removed prior to isolation. The tissues were cultured on potato dextrose agar (PDA) at 28°C for 5 days. A total of 17 unknown strains were isolated; among them, three fungal isolates-designated J11, J12, and J13-exhibited similar morphological characteristics. Colonies appeared white with sparse pigmentation that ranged from ginger-yellow to light yellowish-brown on their reverse side. The conidiomata presented as structures that could be solitary or aggregated; they appeared globose to subglobose in shape and varied from embedded to semi-immersed while exhibiting dark brown to black coloration. The fungi produced aseptate, hyaline α-conidia that are smooth and ellipsoidal in shape; these conidia often arranged dichotomously, measuring ~4.4–3.8 × 2.0–1.8 μm (n = 10 per isolate). No β-conidia were observed. The isolates closely resembled Diaporthe tulliensis in morphology (Tennakoon et al. 2021). For further identification, the internal transcribed spacer (ITS), β-tubulin (TUB), and translation elongation factor 1-α (TEF-1α) regions were amplified using the primer pairs ITS1/ITS4, Bt2a/Bt2b, and EF1-728F/EF1-986R (Slippers et al. 2003), respectively. BLAST analysis of isolates J11 (ITS: PV932116; EF1-α: PX262143; TUB: PX021334), J12 (ITS: PV932117; EF1-α: PX262144; TUB: PX021335) and J13 (ITS: PV932118; EF1-α: PX262145; TUB: PX021336) revealed identities ranging from 98.22% to 98.73% for ITS, from 98.04% to 98.60% for EF1-α, and a high identity of 99.42% for TUB when compared with D. tulliensis (ITS:KR936130; TUB:KR936132; EF1-α:KY569388) (Hsu et al. 2023). A maximum likelihood phylogeny analysis of concatenated ITS, TEF-1α and TUB sequences placed isolates J11-J13 within a clade containing D. tulliensis. Combined with morphological data, these isolates were identified as D. tulliensis. To confirm pathogenicity, one healthy lab-grown pitaya plant was inoculated with 20 mL of a conidial suspension (105 conidia/mL), while another was treated with sterile water as the control (CK). All plants were maintained in a greenhouse at 28°C for 14 days. No symptoms was observed in CK, however, stem rot developed in the treated plants. This experiment was replicated three times with consistent results. To fulfill Koch’s postulates, D. tulliensis was reisolated from symptomatic tissue and confirmed by morphology and sequencing; no fungus were isolated from CK. Reports of D. tulliensis have emerged from Puerto Rico (Serrato-Diaz et al. 2023), Guangxi (Pan et al. 2024) and Taiwan (Hsu et al. 2023). This study represents the first report of D. tulliensis induced stem rot on dragon fruit in Guangxi and provides confirmation that D. tulliensis is responsible for stem rot in pitaya within this region, potentially leading to yield losses ~10–20% in the Long'an demonstration area. The identification paves the way for developing effective disease control strategies.