First report of <i>Fusarium lateritium</i> Causing Postharvest Rot on Bingtang Orange ( <i>Citrus sinensis</i> cv. Dahong Cheng) in Guizhou, China

Bingtang orange (Citrus sinensis cv. Dahong Cheng) is a locally important citrus cultivar in China, valued for its high fruit quality and sweetness. In April 2025, postharvest decay symptoms, characterized by darkened peel and gray-white lesions, were observed on the fruit surface in Huaxi district, Guiyang city, Guizhou province, China (26°24'51.8"N, 106°39'59.8"E). Pathogen isolation was conducted using the tissue isolation method. Symptomatic fruits were washed with distilled water and air-dried. Peel fragments (5 × 5 mm) from the lesion margins were surface-sterilized by immersion in 75% ethanol for 30 s, followed by treatment with 2% NaClO for 30 s. The fragments were then rinsed three times with sterile distilled water and air-dried. The tissue segments were placed on potato dextrose agar (PDA) and incubated at 25°C for 5 d. Hyphal tips emerging from the tissue segments were transferred to fresh PDA plates to obtain pure cultures. The colonies were white on the surface with an orange reverse and exhibited a mean radial growth rate of 4.83 ± 0.27 mm/d on PDA. Macroconidia were sickle-shaped, measuring 26.20 ± 3.30 × 4.14 ± 0.18 μm (mean ± SD; n = 20) with 2–3 septa. Microconidia or chlamydospores were not observed. The morphological characteristics resembled those of Fusarium lateritium as described by Yun et al. (2013) with slightly curved, apical cell hook-shaped or beaked hyaline macroconidia (22.9–29.5 × 3.96–4.35 μm). Genomic DNA was extracted from the mycelia using a DNA Extraction Kit (Biomiga, San Diego, CA, USA). The internal transcribed spacer (ITS) region, translation elongation factor 1-alpha (TEF-1α), and RNA polymerase II second large subunit (RPB2) were amplified using primer pairs ITS4/ITS5 (Cavalcanti et al. 2020), EF1-728F/EF1-986R (O’Donnell et al. 2022), and RPB2-5F2/fRPB2-7cR (Zeng et al. 2023), respectively. The sequences were submitted to GenBank (ITS: PX252053–PX252055, TEF-1α: PX529598–PX529600, and RPB2: PX529601–PX529603). BLASTn search results showed that the ITS, TEF-1α, and RPB2 sequences exhibited 99–100% identity to F. lateritium. Phylogenetic analysis was conducted using Maximum Likelihood (ML) and Bayesian Inference (BI) based on concatenated sequences of the three loci. The isolates (GUCC 25-0097, GUCC 25-0098, and GUCC 25-0099) clustered within the F. lateritium clade with 100% ML bootstrap support and a Bayesian posterior probability of 1.0. Pathogenicity was confirmed through Koch’s postulates. A spore suspension (1 × 106 spores/mL) was prepared, and 20 μL was inoculated onto eight healthy Bingtang orange fruits, of which four were wounded prior to inoculation, and four were left unwounded. Control fruits were treated with sterile water. All fruits were incubated at 28 ± 1°C in the dark at 90% relative humidity. Five days post-inoculation, only the wounded, inoculated fruits developed symptoms identical to those observed in naturally infected fruits. Unwounded and control fruits remained asymptomatic. The pathogen was successfully reisolated from symptomatic tissues, and its morphology and molecular characteristics confirmed its identity as F. lateritium, thereby fulfilling Koch’s postulates. Fusarium lateritium had previously been reported to cause fruit rot on Prunus salicina var. taoxingli in China and gray necrosis in hazelnut in Italy (Vitale et al. 2011; Zeng et al. 2023). Although the species has been recorded on Citrus sinensis in China, previous reports lack detailed provincial-level geographic distribution data and do not address its association with postharvest fruit diseases (Tai 1979). This study represents the first report of F. lateritium causing postharvest rot on Citrus sinensis cv. Dahong Cheng in Guizhou province, China, provides new information on the cultivar-specific host association, distribution, and pathogenicity of this species on citrus.