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|a Begonia semperflorens (Begoniaceae), native to South America, is widely grown in the tropics for vibrant, lasting blooms. In July 2021 and 2022, a leaf rot disease was observed on B. semperflorens in nurseries in Nanning, Guangxi Province, China. The incidence of the disease across the survey area (0.27 ha) was ~35% (n=150). The disease initially appeared as irregular brown water-soaked spots that expanded into grayish-white lesions with brown margins, leading to leaf softening and wilting. To isolate the pathogen, leaf tissues (~ 5 mm2) from lesion borders (18), surface disinfected in 1% NaOCl for 2 min, rinsed thrice in sterile water, plated on potato dextrose agar (PDA), and incubated at 28°C for 3 days (12 h photoperiod). A total of 22 fungal isolates with similar morphological characteristics were obtained and purified via the single-spore isolation method. On PDA, colonies were flat with dense white aerial mycelia, white. On synthetic nutrient poor agar (SNA), colonies were convex with sparse aerial mycelia. Microconidia on SNA were oval to subcylindrical, smooth, thin-walled, hyaline, 0-1 septate, measuring 5.8-11.7 × 2.2-4.5 μm (n=60). Macroconidia were falcate, slender, straight to curved, hyaline, apical cell papillate to hooked, basal cell barely to distinctly notched, 3 to 6 septate, measuring 42.4 to 63.7 × 3.7 to 5.9 μm (n = 60). Chlamydospores were smooth-walled and globose to subglobose. Conidiophores were straight or curved, smooth, thin-walled, mostly simple, with single monophialides. The internal transcribed spacer (ITS), partial translation elongation factor-1 alpha (TEF-1α), and RNA polymerase second largest subunit (RPB2) genes of the representative isolate HT-2D were amplified and sequenced using the primer pairs ITS1/ITS4 (White et al. 1990), EF-1/EF-2 (O'Donnell et al., 1998), and 5f2/11ar (Liu et al. 1999, Reeb et al. 2004). The ITS (OQ048269) and RPB2 (OP994263) sequences showed 99.3% (532/536 bp) and 99.4% (832/837 bp) identity with those of Fusarium parceramosum CBS 115695 (JX435199 and JX435249, respectively), and TEF-1α (OP994261) showed 100% (662/662 bp) identity with the corresponding locus of F. parceramosum NRRL 31158 (DQ246916) (Sandoval-Denis et al., 2019). Furthermore, multilocus phylogenetic analysis based on ITS, TEF-1α, and RPB2 confirmed that isolate HT-2D clustered with F. parceramosum. Thus, based on morphological and molecular characteristics, the isolates were identified as F. parceramosum (synonym: Neocosmospora parceramosa) (Guarnaccia et al., 2022; O'Donnell et al., 2020; Sandoval-Denis et al., 2019). To test pathogenicity, three healthy leaves on each of three B. semperflorens plants were inoculated with a 10-μl HT-2D conidial suspension (1 × 106 conidia/mL). Three control leaves were treated with sterilized water. All plants were incubated in a greenhouse (26°C, 12 h photoperiod, ~ 90% humidity) within plastic bags. The experiments were repeated three times with three replicates in each. Three days after inoculation, B. semperflorens leaves exhibited irregular lesions with pale yellowish-brown edges, which aligned with field observations, while the control plants remained healthy. To fulfill Koch's postulates, the F. parceramosum isolates were consistently re-isolated from the infected leaves and confirmed by morphology and sequencing (ITS, TEF-1α, and RPB2), with no fungi detected in controls. This is the first report of F. parceramosum causing leaf rot on B. semperflorens
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