First Report of Leaf Spot Caused by Curvularia asiatica on Water Spinach ( Ipomoea aquatica ) in the United States

Water spinach (Ipomoea aquatica Forsk.) is a species in the Convolvulaceae family and a leafy vegetable commonly cultivated in Asia, particularly in China and India (Hao et al. 2021). In Florida, USA, it is grown as a specialty crop in moist soil or hydroponic systems. In November 2024, water spinach plants with dark leaf spots from a registered farm were observed and maintained in a greenhouse for study. In February 2025, irregular brown leaf spots with a yellow halo were observed (≈60% incidence, 15% severity). Small leaf sections (0.5 cm) were excised, surface-sterilized in 75% ethanol for 30 s and 1% NaClO for 1 min, then rinsed twice with sterile water, air dried, plated on 1% water agar, and incubated at 25°C in the dark. After six days, mycelia emerging from tissue were subcultured on PDA and incubated for seven days. Colonies were velvety to woolly, olivaceous gray to brown. Conidiophores were mostly straight, unbranched, brown, septate, and apically geniculate, with terminal conidiogenous cells. Conidia were typically curved, ellipsoidal, 15.3–28.4 (22.8 ± 3.3) × 7.2–12.6 (10.2 ± 1.3) µm (n = 40), with 3–4 septa, a hyaline apical cell, and an enlarged dark brown to black, sometimes thick-walled central cell. Morphology was consistent with Curvularia asiatica (syn. C. asianensis) (Manamgoda et al. 2012). Genomic DNA was extracted from WY6 using the Quick-DNA Fungal/Bacterial Miniprep Kit (Zymo Research). ITS, GAPDH, and TEF1 were amplified and sequenced with primers ITS1/ITS4 (White et al. 1990), gpd1/gpd2 (Berbee et al. 1999), and EF1-983/EF1-2218R (Schoch et al. 2009). Sequences were deposited in GenBank as PX555836 (ITS), PX693380 (GAPDH), and PX693379 (TEF1). ITS showed 100% identity to C. millisiae (OK661031), whereas GAPDH and TEF1 matched C. asiatica (100% MN264083; 99.8% MK886804). Because ITS has limited resolution in Curvularia, multilocus phylogeny (ITS+TEF1+GAPDH) placed WY6 within the C. asiatica clade, with C. senegalensis as the closest relative. However, C. senegalensis has intercalary conidiogenous cells and larger, 3–5 septate conidia (22–31 × 10–14 µm) (Spegazzini, 1914; Guarro et al., 1999), whereas C. asiatica has only terminal conidiogenous cells and smaller conidia with ≤4 septa, consistent with our observations. Therefore, based on morphology and phylogeny, WY6 was identified as C. asiatica. Pathogenicity tests were conducted on water spinach in a greenhouse at 28–32°C. Inoculum of C. asiatica WY6 was prepared by suspending conidia from 8-day PDA cultures in sterile water and adjusted to 1 × 106 conidia/mL. Five plants were sprayed with 50 mL of spore suspension, and five controls plants received sterile water. Plants were bagged for 48 h to maintain humidity. Five days after inoculation, four of the five plants developed brown lesions with a yellow halo, like the original symptoms, while controls remained healthy. The pathogenicity test was repeated twice with similar results. Koch’s postulates were fulfilled by reisolating C. asiatica from all symptomatic leaves, confirmed morphologically. To our knowledge, this is the first report of C. asiatica infecting water spinach (I. aquatica). This species has been reported as a saprobe on grasses (Panicum spp.), sugarcane and rice, and as a pathogen on Epipremnum pinnatum (Manamgoda et al. 2012) and Sansevieria trifasciata in Malaysia (Kee et al. 2020). This finding expands the host range and distribution of C. asiatica and highlights the need for management strategies in water spinach.