First Report of <i>Alternaria alternata</i> Causing Leaf Blight on Garlic in the Republic of Korea

Garlic (Allium sativum L.) is one of the most widely used and economically significant spice crops worldwide. However, it is vulnerable to several diseases, including Stemphylium leaf blight caused by S. vesicarium and purple blotch disease caused by Alternaria porri. In May 2024, leaf blight symptoms were observed on garlic plants in Gyeonggi Province, Republic of Korea. Diseased leaves exhibited dry symptoms, progressing from the tip towards the center, with brown to black sporodochia visible on the surface. To isolate the pathogens responsible for these symptoms, symptomatic leaf samples were collected from garlic plants grown on farms in Icheon and Anseong-si, Gyeonggi Province, Republic of Korea. The diseased leaves were surface-sterilized in 1% sodium hypochlorite for 1 min, rinsed with sterile water, placed on potato dextrose agar (PDA) containing 100 µg/ml of streptomycin and ampicillin, and incubated at 25℃ for 5 days. Four fungal isolates (SSV2, SSV4, SSV6, and SSV15) were obtained and pure-cultured using the single-spore isolation method. These isolates produced brown multi-celled conidia with obclavate to obpyriform shape on PDA, consistent with the characteristics of Alternaria alternata as previously described (Yu et al., 2016; Zhu & Xiao, 2015). Genomic DNA was extracted from each isolate, and the internal transcribed spacer (ITS) region of the ribosomal DNA and the translation elongation factor-1α (tef1) were amplified using primer pairs ITS4/ITS5 (White et al., 1990) and EF1-728F/EF-2 (Carbone & Kohn, 1999), respectively. The resulting sequences were deposited in GenBank (accession No. PV426258-PV426261 and PV440513-PV440516). Maximum likelihood phylogenetic analyses were performed using the GTR + GAMMA + I model with 1000 bootstrap replicates in RaxML (Edler et al., 2021). Based on the combined ITS + tef1 dataset, the four isolates clustered with the same clade as reference strains of A. alternata, supported by a bootstrap value of 100%. To assess pathogenicity, garlic plants (cv. Dae-Seo) at the four-leaf stage were inoculated by spraying with a spore suspension (1 × 10⁶ spores/ml). The plants were kept in a 100% humidity condition for 24 hours and then transferred to a growth chamber at 25°C with a 12-hour photoperiod. Five days after inoculation, chlorosis and necrosis developed on the first through third leaves of inoculated plants. Although disease severity varied among isolates, all four isolates were capable of causing typical leaf blight symptoms. To fulfill Koch's postulates, symptomatic leaves were placed on PDA for 7 days. The original fungal strains were successfully re-isolated and confirmed as A. alternata through ITS sequencing. To our knowledge, this is the first confirmed report of garlic leaf blight caused by A. alternata in the Republic of Korea.