Leaf blight of Photinia × fraseri caused by Seiridium guangyuanum in China

In October 2023, severe leaf blight symptoms were observed on Photinia × fraseri at the campus of Jiangxi Agriculture University, Nanchang City, Jiangxi Province, China, with a disease incidence of around 5%. Symptoms initially appeared as small, irregular brown spots with black margins that subsequently enlarged into larger, irregular necrotic areas. To isolate the causal agent, symptomatic leaves were randomly collected and cut into 5 × 5 mm pieces. The pieces were surface-disinfected in 70% ethanol for 30 s, followed by 1% NaClO for 30 s, rinsed three times in sterile distilled water, and dried on sterile filter paper. They were then placed onto potato dextrose agar (PDA) plates and incubated at 25 ℃ in the dark. Ten isolates with similar morphological characteristics were obtained. Colonies on PDA were not very dense, circular or irregular, flattened to slightly raised. The colony front was creamy and white at the margin, while the reverse was tangerine. Conidia were lunate to falcate, straight or slightly curved, five-septate, with appendages, measuring 21.8–36.4 × 7.5–9.9 μm (mean: 26.5 × 8.6 μm, n=30). Based on these morphological features, the pathogen was tentatively identified as Seiridium spp. To identify the fungus at species level, genomic DNA of three representative isolates (JFRL 03-3105, JFRL 03-3106 and JFRL 03-3107) was extracted using the CTAB method (Conlon et al. 2022). The ITS region, RPB2, TEF1-ɑ and TUB2 genes were amplified and sequenced using primers ITS5/ITS4, fRPB2-5f2/fRPB2-7cr, EF1-728F/EF2, and T1/Bt2B, respectively (Bonthond et al. 2018; Razaghi et al. 2024). Sequences were deposited in GenBank under accession numbers PX136231-PX136233 (ITS), PX227337-PX227339 (RPB2), PX204794-PX204796 (TEF1-ɑ), and PX204791-PX204793 (TUB2). A BLASTn searches of the obtained sequences revealed 99%-100% identity to corresponding loci from the type strain Seiridium guangyuanum CGMCC 3.23561. Maximum-likelihood analysis of the combined ITS, RPB2, TEF1-α and TUB2 dataset was performed in PhyloSuite v1.2.2 (Zhang et al. 2020). The resulting phylogeny placed the three isolates in a strongly supported clade with well-annotated strains of S. guangyuanum. Thus, the fungus was conclusively identified as S. guangyuanum on the basis of both morphological and molecular data. To confirm pathogenicity, six healthy leaves of two 3-year-old potted Photinia × fraseri were surface sterilized, wounded, and inoculated with 20 µl of conidial suspension (106 conidia/ml) of the isolate JFRL 03-3105, while another set of six wounded leaves were inoculated with sterile distilled water as controls. All plants were grown in a climate chamber at 25℃ and 85% relative humidity, and the experiment was repeated twice. After four days, the leaves inoculated with S. guangyuanum exhibited symptoms similar to those observed in field, whereas the control leaves remained symptomless. S. guangyuanum was consistently re-isolated from the symptomatic leaves and confirmed by morphological and molecular analyses, thereby fulfilling Koch’s postulates. S. guangyuanum has previously been reported as the causal agent of branch diseases on Vernicia fordii, Camellia oleifera and Olea europaea in China (Li et al. 2022). To the best of our knowledge, this is the first report of S. guangyuanum causing leaf blight on Photinia × fraseri in China, thereby expanding the known host range of this fungus.