First Report of <i>Diplodia seriata</i> Associated with Canker and Dieback of Korla Pear ( <i>Pyrus sinkiangensis</i> ) in Xinjiang, China

Korla pear (Pyrus sinkiangensis Yü) is an important economic fruit crop in Xinjiang, China. In October 2023, a field survey was conducted in four commercial Korla pear orchards located in Alar, Xinjiang (40°33′21″ N, 81°10′38″ E), comprising approximately 200 trees in total. About 23% of the trees exhibited symptoms of bark cracking and canker lesions (3–8 cm in length) accompanied by vascular discoloration, branch dieback, and in severe cases, tree death. Ten diseased branch samples were collected. Necrotic tissues were excised with a sterile scalpel, surface-sterilized in 1% sodium hypochlorite followed by 75% ethanol, rinsed five times with sterile distilled water, and plated onto potato dextrose agar (PDA). Plates were incubated at 25 °C in the dark for 48 h, then under a 12 h light/12 h dark cycle for a total of 7 days. Three representative isolates (ALEMLL-06A, ALEWBG-8B, and ALEWXJ-7) were selected from diseased Korla pear branches showing similar symptoms and colony characteristics for subsequent identification and pathogenicity tests. Conidia were ellipsoid with rounded ends, aseptate to single-septate, measuring 23.4–29.6 × 10.2–12.8 µm (mean = 26.1 × 11.4 µm; n = 50). Based on morphological traits, the isolates were tentatively identified as Diplodia seriata. Genomic DNA was extracted, and the ribosomal internal transcribed spacer (ITS), β-tubulin (TUB2), and translation elongation factor 1-alpha (EF1-α) regions were amplified using the primer sets ITS1/ITS4 (White et al., 1990), Bt2a/Bt2b (Glass & Donaldson, 1995), and EF1-728F/EF1-986R (Carbone & Kohn, 1999), respectively. The resulting sequences were deposited in GenBank under accession numbers ITS: PV785237, PV785238, PV785239; TUB2: PV815595, PV815596, PV815597; and EF1-α: PV945469, PV945470, PV945471. BLAST analysis showed >98% similarity with reference sequences of D. seriata DS-01 (ITS: KT447248; TUB2: KT460190; EF1-α: KT460188). Sequences of ITS, TUB2, and EF1-α were aligned and concatenated using MEGA 7.0, and a neighbor-joining phylogenetic tree was constructed based on the Tamura-Nei + G model with 1,000 bootstrap replicates. Pathogenicity tests were conducted on healthy 1-year-old shoots of 15-year-old Korla pear trees in Alar. For each isolate, five shoots (one per tree) were inoculated with 4-mm-diameter mycelial plugs from 3-day-old PDA cultures, and five shoots received sterile PDA plugs as controls. Inoculation sites were sealed with Parafilm, and trees were kept under natural orchard conditions. Typical canker and dieback symptoms developed 5 days after inoculation, whereas controls remained symptomless. The pathogen was reisolated from inoculated branches showing typical symptoms, and its morphology was identical to the original isolates, confirming that Koch’s postulates were fulfilled. Previously, D. seriata has been reported as a pathogen of loquat, cedar, apple, olive, and Cotoneaster salicifolius (Palou et al. 2013; Jiao et al. 2014; Lódolo et al. 2022; Kaliterna et al. 2012; Sha et al. 2023). To our knowledge, this is the first report of pathogenic D. seriata on P. sinkiangensis worldwide. This finding provides a basis for future studies on the epidemiology and distribution of D. seriata in Korla pear orchards.