First report of Agroathelia rolfsii causing Southern blight on Staurogyne repens in Florida, U.S.A.

Staurogyne repens (Nees) Kuntze (Acanthaceae), known as repens, has recently gained commercial value, particularly for use in landscaped aquariums (Sereda et al. 2016). In August 2024, about 50% of repens in a nursery were stunted and showing necrotic tissue along the stems and roots (FDACS-DPI LIST 08292024-8726 and 11062024-10975). During root examination, sclerotia‑like structures were placed on acidified potato dextrose agar (APDA) and incubated at room temperature for four days (following Urbina et al. 2024), consistently yielding white mycelial colonies. In culture, the isolated fungal strain produced abundant white mycelium with hyphae (3 to 6 µm wide) forming clamp connections; and cream to light brown sclerotia of 1.3 to 3.7 mm wide. The dried culture was deposited in the Plant Industry Herbarium Gainesville (PIHG 19741). For molecular identification, DNA was extracted from mycelium using the DNeasy Plant Mini Kit (Qiagen) and used for PCR. The internal transcribed spacer region ITS (White et al. 1990), the large subunit ribosomal ribonucleic acid (LSU), the elongation factor 1α (tef1) and the RNA polymerase II subunit (rpb2) (Vilgalys and Hester 1990; White et al. 1990; Lee et al. 2025) were sequenced and deposited in GenBank (acc. no. ITS-LSU: PX070273, tef1: PX921696, rpb2: PZ242900). In concordance with phylogenetic analyses, MegaBLAST searches showed high sequence similarity to previously reported conspecific strains of Agroathelia rolfsii (Sacc.) Redhead & Mullineux (Amylocorticiaceae) (ITS-LSU (PZ177406, 98.66%, 1182/1198), tef1 (MN702785, 99.81%, 528/529), and rpb2 (OR192511, 99.09%, 650/656)) as well as strong similarity to the whole genome of strain CCF1 (JAXRUS01: ITS-LSU (98.75%, 1187/1202), tef1 (99.44%, 534/537), and rpb2 (98.33%, 707/719)) (You et al. 2025). Pathogenicity test was conducted using tissue cultured repens seedlings obtained from a commercial retail establishment. The experiment included 15 replicates and five controls. Seedlings were individually planted in 50 ml Falcon tubes containing sterile perlite with roots submerged in water, lids partially opened to ensure adequate air circulation and grown under a 12-hour photoperiod on a light shelf, at 20 °C. Five-day-old A. rolfsii cultures, grown in APDA were used to inoculate repens seedlings. A 5-mm agar plug from the edge of an actively growing culture was placed on the stem of each seedling close to the substrate. Control seedlings were inoculated with a sterile, non-colonized agar plug. The experiment was repeated once. After 12 days of inoculation, symptoms were evaluated and pathogen was re-isolated by placing sclerotia that developed on the plant in APDA. DNA was extracted and the ITS region was sequenced using the same methods mentioned above. All inoculated plants were symptomatic, resembling the symptoms found on the nursery plants and produced 11 ± 5 sclerotia per treatment on average, while controls remained healthy. Agroathelia rolfsii, the cause of Southern blight, affects more than 260 plant genera (Farr et al. 2024). Although common in terrestrial systems, it has rarely been detected in aquatic plants. Nevertheless, it was reported for the first time on water lily in 2024 (Urbina et al. 2024). This shift from land to aquatic plants could be driven by its adaptability, broad host range and efficient dispersal mechanisms. To our knowledge, this is the first time that A. rolfsii is reported on Staurogyne repens in Florida and elsewhere.