First Report of Bacterial Blight of Hazelnut ( <i>Corylus heterophylla</i> ) Caused by <i>Xanthomonas arboricola</i> pv. <i>corylina</i> in South Korea

Hazelnut (Corylus heterophylla Fisch. Ex Trautv.), a member of the family Betulaceae, is a common tree with economic potential as a nut crop. Recently, there has been growing interest for its cultivation in South Korea. In July 2023, bacterial blight symptoms on leaves were observed on hazelnut trees in Bonghwa, (36°96'29.9"N 128°73'01.2"E), and similar symptoms were observed in Yeoju in June 2024 (37°18'64.6"N 127°69'14.5"E). Numerous small, irregular brown spots (1–2 mm diameter) with occasional chlorotic halos were observed on leaves. The disease incidence was approximately 50%, with disease severity varied between 50% to 80%, based on visual assessment of individual leaves. Leaf samples were collected from infected trees, and surface sterilized with 1% sodium hypochlorite for 30 s, 70% ethanol for 30 s and rinsed with sterile distilled water (SDW) three times. Small portions of leaf tissue were excised from the margin of the lesions, macerated in 1 ml of SDW and incubated at room temperature for 30 min. The suspension was serially diluted, then streaked onto nutrient agar (NA) medium. After 48 h incubation at 25℃, mucoid, circular and yellow colonies (Kałużna et al. 2021) on NA. For phylogenetic analysis, genomic DNA was extracted from two isolates (NIFOS 202307-B01 and NIFOS 202406-B02). For molecular identification, the multilocus sequence analysis (MLSA) was performed using partial sequences of six housekeeping genes - atpD (ATP synthase beta subunit); dnaK (heat shock protein 70-kDa); efP (elongation factor P); gyrB (DNA gyrase subunit B); fyuA (TonB-dependent receptor), and rpoD (RNA polymerase sigma-70 factor), following previously described protocols (Boudon et al. 2005; Fargier et al. 2011). Alignment was performed by MAFFT ver. 7.526. (Katoh and Standley. 2013), and tree construction was conducted using RAxML-NG v.1.2.2. (Kozolov et al., 2019). Through maximum-likelihood approach, the isolates were mostly closely related to X. arboricola pv. corylina. The sequences were deposited in NCBI GenBank (accession nos. PV754461 and PV754462 for atpD, PV754463 and PV754464 for dnaK, PV754465 and PV754466 for efP, PV754467 and PV754468 for gyrB, PV754469 and PV754470 for fyuA, PV754471 and PV754472 for rpoD). To confirm Koch’s postulates, the inoculation test was performed on hazelnut trees. Three branches with four to five healthy leaves were sprayed with the bacterial suspensions of isolate NIFOS 202307-B01 (OD600 = 0.2, approx. 1 × 108 CFU/ml), with SDW as a negative control. The inoculated branches were incubated under plastic bags for two weeks, to keep high humidity conditions. After two weeks, brown spots and chlorotic halos were observed on the inoculated leaves, while control leaves were symptomless. The bacterium was re-isolated from the symptomatic leaves, and identified as the same species, fulfilling Koch’s postulates. This is the first report of X. arboricola pv. corylina causing bacterial blight of hazelnut in South Korea. Our finding highlights the need for further research on disease management strategies to mitigate the impact on hazelnut cultivation.