First Report of Leaf Blight on Sweet Potatoes Caused by Fusarium graminearum in China

Sweet potato (Ipomoea batatas (L.) Lam.) is an important food and industrial crop widely cultivated in China. In October 2024, severe leaf blight was observed in three commercial sweet potato fields in Yuangezhuang Town, Laishan District, Yantai City, Shandong Province, China (37.30°N, 121.38°E). Disease incidence, calculated as the percentage of symptomatic plants among approximately 100 plants surveyed per field, averaged 85.3% across the three fields. Symptoms primarily initiated along the leaf margins as small, irregular brown spots surrounded by distinct chlorotic halos. Subsequently, these lesions gradually coalesced into larger necrotic areas, with peripheral yellowish halos expanding outward. At the late disease stage, the entire leaf lamina turned brown and necrotic (~100% of leaf area), followed by leaf curling. Surface-disinfected (75% ethanol for 90 s, 1% NaClO for 180 s) lesion margins (5 × 5 mm, n=20) from ten diseased samples were directly placed on PDA amended with rifampicin (50 µg/mL) and kanamycin (50 µg/mL) at 25°C in the dark for 3 days. A total of 17 fungal isolates were recovered, of which 10 displayed identical morphology. Representative isolate DLV4-2 was purified using the single-spore method. On PDA, colonies were initially white with fluffy aerial mycelium, later developing central pinkish pigmentation. After 10 days on mung bean broth (MBB) (Constantin et al. 2020), hyaline, falcate, 3–5 septate macroconidia were observed, measuring 36.7–60.4 × 3.5–4.9 μm (avrSD= 48.3± 5.59 × 4.2± 0.36 μm, n=20). Genomic DNA of isolate DLV4-2 was extracted. The ITS region, EF1-α, BT2a, and RPB2 gene fragments were amplified using primers ITS1/ITS4, EF1/EF2 , BT2a/BT2b, and RPB2-5F2/RPB2-7cR (Schoch, C. L., et al. 2012，Glass and Donaldson 1995，O'Donnell et al. 1998), respectively. Sequences were deposited in GenBank (accession nos. ITS: PZ225278, EF1-α: PZ269964, BT2a: PZ269963, and RPB2: PZ513995). BLASTn analysis against GenBank and the Fusarium MLST database assigned DLV4-2 to the Fusarium graminearum species complex (FGSC), with 100% ITS identity. Further maximum likelihood (ML) phylogenetic analysis based on concatenated EF1-α, BT2a, and RPB2 sequences was performed in MEGA 11, which clustered the isolate within F. graminearum. Pathogenicity was confirmed by inoculating scratched leaves of 10 healthy I. batatas cv. Yanshu 25 seedlings with 30 µL of conidial suspension (1 × 105 conidia/mL); control leaves were treated with sterile water. All plants were maintained in a greenhouse at 25°C and 70% relative humidity under natural light. After 11 days, inoculated leaves developed brown necrotic lesions with chlorotic halos at the inoculation sites, whereas controls remained asymptomatic. The same fungus was consistently re-isolated and reidentified, from symptomatic lesions, fulfilling Koch’s postulates. Although best known as a cereal pathogen, F. graminearum has also been reported on dicot crops in China, including tomato and Panax notoginseng (Cai et al. 2023; Ye et al. 2020). To our knowledge, this is the first report of F. graminearum causing leaf blight on sweet potato in China. This finding provides important information for managing sweet potato leaf blight.