First Report of Leaf Spot Caused by Alternaria alternata on Rice in Korea

Rice (Oryza sativa L.) is the most important cereal crop in Korea. In 2025, a survey of symptomatic rice plants in Haenam, Jeollanam-do, recovered Alternaria spp. at a high frequency. The isolates were not identified to species level, and pathogenicity was not examined (Kim et al. 2025). Leaf spot symptoms were observed in July 2025 in a rice field in Haenam, Jeollanam-do, Korea. Approximately 5% of plants showed symptoms. Lesions initially appeared as small dark-brown flecks that enlarged into circular or irregular spots with dark margins and pale gray to brown centers. As lesions expanded, adjacent lesions frequently coalesced and caused extensive blighting. Small pieces of tissue excised from lesion margins were surface-disinfested, plated on water agar amended with streptomycin (100 μg/ml), and incubated at 25°C. Thirty fungal isolates were recovered, and three representative Alternaria isolates (SYP-2629, SYP-2649, and SYP-2659) were selected for further analyses. Colonies grown on PDA for 14 days were dark olive-brown in the center with grayish margins. Conidia were light brown, ellipsoid to obclavate, with roughened walls and 1–5 transverse and 0–3 longitudinal septa, measuring 11.9–56.0 × 5.9–16.4 μm (mean 25.6 × 10.9 μm, n = 100). These characteristics agreed with published descriptions of A. alternata (Woudenberg et al. 2015). The ITS, gapdh, tef1, rpb2, Alt a1, endoPG, and OPA10-2 loci were amplified with primers (White et al. 1990; Woudenberg et al. 2013, 2015). Sequences were deposited in GenBank as PX694587, PX694590, and PX694598 (ITS), PZ576500–02 (gapdh), PZ576503–05 (tef1), PZ576506–08 (rpb2), PZ576509–11 (Alt a1), PZ576512–14 (endoPG), and PZ576515–17 (OPA10-2). BLASTn searches showed 100% identity with the ex-type strain CBS 102598 of A. alternata for ITS, tef1, rpb2, and Alt a1. Sequence identities of gapdh, endoPG and OPA10-2 were 99.65%, 99.33% and 97.31%, respectively. Maximum-likelihood analysis of the combined seven-locus dataset using MEGA X grouped all three isolates with reference isolates of A. alternata. Isolate SYP-2649 was used for pathogenicity tests. A conidial suspension (1 × 10⁶ conidia/ml) prepared from 14-day-old PDA cultures was sprayed onto wounded and unwounded leaves of five-leaf-stage rice plants. Plants were maintained under greenhouse conditions (18–26°C, 75–90% relative humidity). Leaf spot symptoms identical to those observed in the field developed 14 days after inoculation. Control plants remained symptomless. The fungus was consistently re-isolated, and the identity of the re-isolates was confirmed by sequencing the tef1 locus. The experiment was repeated three times with similar results. To our knowledge, this is the first report of A. alternata causing leaf spot of O. sativa in Korea. Alternaria species have previously been associated with leaf spot diseases of rice, including A. arborescens on cultivated rice in Pakistan (Akhtar et al. 2014) and A. alternata on wild rice (Oryza rufipogon) in China (Zhong et al. 2022). In Korea, A. alternata has previously been detected from discolored rice seeds (Shim et al. 2005), but not from leaf spot lesions of rice. This report extends the known distribution of A. alternata on rice and provides a basis for accurate diagnosis and disease surveillance in Korean rice production.