First Report of Root Rot and Stem Blight in Water Spinach ( Ipomoea aquatica ) Caused by Ceratobasidium sp. Related to AG‐A and AG‐K Lineages in Florida, USA

Water spinach (Ipomoea aquatica) is an aquatic vegetable in the Convolvulaceae family which is widely consumed in Asia. In Florida, its cultivation requires a permit from the Florida Department of Agriculture and Consumer Services due to its invasive potential. In June 2024, root rot and stem blight were observed on water spinach (cv. Ching Quat) at a registered farm in Indian River County, Florida, affecting c. 50% of the crop. Symptoms began as small brown lesions at the stem base, expanding above and below the soil line, causing necrosis and dieback. Diseased stems were cut into small pieces (c. 5 mm), surface disinfested in 70% ethanol for 30 s followed by 0.75% NaClO for 2 min, rinsed three timesin sterile water, plated on 1% water agar and incubated at 25°C for 3–5 days. White to light grey fungal colonies were consistently recovered, and 10 isolates were purified on potato dextrose agar (PDA). Two isolates (FT-B and FT-C) were selected for morphological and molecular identification. Colonies were initially white, later turning light brown (Figure 1). Hyphae were septate, hyaline, 5.6–10.2 µm wide (n = 30) and branched at right angles with slight constrictions (Figure 2A). Safranine staining showed two nuclei per cell (Figure 2B). These traits matched Rhizoctonia-like fungi, indicating placement within the binucleate Rhizoctonia group, the anamorph of Ceratobasidium (Sneh et al. 1991). Genomic DNA was extracted with a Quick-DNA Fungal/Bacterial Miniprep Kit (Zymo Research, USA). ITS and TEF1-α were amplified using ITS1/ITS4 (White et al. 1990) and EF1-983F/EF1-1567R (Rehner and Buckley 2005). Sequences were deposited in GenBank as Accession Nos. PQ431974-PQ431975 (ITS) and PX206235-PX206234 (TEF1-α). BLASTn showed 98.4%–98.5% identity to Ceratobasidium ITS sequences without AG designation. Separate ITS and TEF1-α phylogenies placed FT-B and FT-C in a strongly supported Ceratobasidium lineage closely related to AG-A and AG-K but distinct from both (Figure 3). Pathogenicity tests were performed on five 2-week-old water spinach (cv. Ching Quat) plants per isolate by placing 5-mm colonised PDA plugs at the stem base and maintaining high humidity by bagging the plants. Five control plants received only sterile plugs. Inoculated plants developed stem blight within 4 weeks, while controls remained symptomless (Figure 4). Ceratobasidium sp. was reisolated from all symptomatic plants (5/5 per isolate) and was confirmed based on microscopic characteristics matching with the inoculated isolates, fulfilling Koch's postulates. The pathogenicity test was repeated three times with consistent results. Ceratobasidium spp. include pathogenic lineages reported on multiple crops including potato and peanut (López-Corrales et al. 2024; Li et al. 2025). To our knowledge, this is the first report of root rot and stem blight of water spinach caused by Ceratobasidium sp., expanding its host range and underscoring the need for management strategies.