First Report of <i>Alternaria alternata</i> Causing Needle Blight on <i>Picea jezoensis</i> in Korea

Picea jezoensis (Siebold & Zucc.) Carrière is an alpine conifer of high ecological and economic value worldwide. However, its populations has been adversely affected by the effects of climate change, including the increased proliferation and infection by fungal phytopathogens. In April 2023, needle blight symptoms (55% disease incidence) were observed on P. jezoensis seedlings in a forest nursery located in Gwangju, South Korea. The symptoms progressed from needle chlorosis to dry brown discoloration. Needles from symptomatic seedlings were collected and transported to the laboratory. For pathogen isolation, the needles were sterilized with 1% sodium hypochlorite (NaOCl), rinsed three times with sterile distilled water, and dried. The dried needle samples were placed on potato dextrose agar (PDA) plates supplemented with 0.05% streptomycin and incubated at 25 °C (Matic et al. 2020). Six morphologically similar fungal isolates were obtained. A representative isolate, PJ9YL1, initially produced white mycelia that gradually turned grayish green from the center as the colony matured. The conidia were brown and obclavate, averaging 23.2 × 9.35 μm (n = 30), with 0-3 vertical and 3-4 horizontal septa, consistent with the characteristics of Alternaria sp. (Kawashimo and Sakurai 2024). To further identify the isolate, multiple gene regions were sequenced: NS1/NS4 (18S rRNA; SSU), LSU1Fd/LR5 (28S rRNA; LSU), ITS4/ITS5 (internal transcribed spacer; ITS), gdp1/gdp2 (glyceraldehyde-3-phosphate dehydrogenase; GAPDH), EF1-728F/EF1-986R (translation elongation factor 1-alpha; TEF1), Alt-for/Alt-rev (Alternaria major allergen gene; Alt a 1), and PG3/PG2b (endopolygalacturonase; endoPG) (Woudenberg et al. 2015). The obtained sequences were deposited in GenBank (SSU: PQ510455, LSU: PQ510456, ITS: PQ510454, GAPDH: LC852693, TEF1: LC852694, Alt a 1: LC852692, and endoPG: LC852230). BLASTn results revealed that the SSU, LSU, ITS, GAPDH, TEF1, Alt a 1, and endoPG sequences of isolate PJ9YL1 had 99.58% (236/237), 99.70% (336/337), 100% (531/531), 99.58% (236/237), 100% (139/139), 100% (123/123), and 99.36% (309/311) respective sequence identity to the ex-type strain A. alternata CBS 916.96 (KC584507, DQ678082, AF347031, AY278808, KC584634, AY563301, and JQ811978) (Zhang et al. 2023). Phylogenetic analysis based on the concatenated sequences (SSU, LSU, ITS, GAPDH, TEF1, Alt a 1, and endoPG) revealed that the isolate PJ9YL1 clustered with A. alternata strains. A pathogenicity test was conducted by spraying a conidial suspension (105 conidia/mL) onto the needles of 10 healthy 1-year-old P. jezoensis seedlings. Ten seedlings sprayed with distilled water served as the negative control. Needle blight symptoms developed two weeks post-inoculation, and severely diseased seedlings exhibited complete needle browning and mortality within one month. The pathogen was isolated from infected needles and confirmed as A. alternata based on morphological features and molecular analysis, thereby fulfilling Koch's postulate. In contrast, seedlings in the negative control group remained asymptomatic. To the best of our knowledge, this is the first report of A. alternata causing needle blight on P. jezoensis in Korea. Given the declining population of P. jezoensis in alpine ecosystems, this study provides crucial information for the conservation and management of this ecologically significant species.