Basal Stem Rot on Water Spinach ( Ipomoea aquatica ) in the United States Caused by a Monilochaetes sp. Closely Related to Monilochaetes dimorphospora

Water spinach (Ipomoea aquatica) is widely cultivated in Asia and grown as a specialty crop in Florida, USA. In June 2024, basal stem rot was observed on cv. Ching Quat in a greenhouse in Fort Pierce, Florida, affecting approximately 50% of plants. Symptoms included brown basal lesions extending above and below the soil line, causing necrosis, branch dieback, and plant decline. Similar symptoms were observed in greenhouse plantings in 2025 and 2026. Diseased stem tissues were plated on potato dextrose agar (PDA). After 5–7 days at 25°C, colonies were dark olivaceous to black with concentric zonation and a velvety appearance (Figure 1). Microscopy revealed septate, pale brown hyphae and phialidic conidiogenous cells producing hyaline, aseptate conidia (Figure 2). Conidia were ellipsoidal to cylindrical, 15.7–30.9 × 5.1–10.1 µm (mean ± SD = 19.5 ± 3.8 × 7.0 ± 1.3 µm, n = 30), consistent with Monilochaetes spp. (Réblová et al. 2011). Genomic DNA was extracted using a Quick-DNA Fungal/Bacterial Miniprep Kit (Zymo Research, USA). ITS (internal transcribed spacer), SSU (small subunit ribosomal RNA), LSU (large subunit ribosomal RNA), and TEF1-α (translation elongation factor 1-alpha) were amplified and sequenced using primer pairs ITS1/ITS4, NS1/NS4, LR0R/LR5, and EF1-983F/EF1-1567R, respectively (White et al. 1990; Vilgalys and Hester 1990; Rehner and Buckley 2005). Sequences were deposited in GenBank under Accession Nos. PZ465971 (ITS), PZ541186 (SSU), PZ466070 (LSU), and PZ495118 (TEF1-α). BLASTn analyses showed the highest identity to Monilochaetes dimorphospora (99.9% LSU, HQ609480; 99.0% SSU, NG062390; 97.0% ITS, NR_137765). The TEF1-α sequence shared a 94.8% identity with M. pteridophytophila (MW829187) but was excluded from phylogenetic analyses because TEF1-α sequences are unavailable for most Monilochaetes spp. Maximum-likelihood analyses of individual and concatenated ITS, LSU, and SSU sequences consistently placed the isolate within the Monilochaetes clade as a sister lineage to M. dimorphospora (Figure 3). Despite the high LSU and SSU identity, lower ITS identity and limited multilocus reference data precluded species-level identification; therefore, the isolate was conservatively identified as Monilochaetes sp. Pathogenicity was assessed on healthy water spinach plants (cv. Ching Quat) under greenhouse conditions. Colonized PDA plugs (3 mm) were placed at the stem base of wounded and non-wounded plants; controls received sterile PDA plugs. Plants were maintained under high humidity for 48 h. Brown to black necrotic lesions developed within 2–3 weeks on inoculated plants, consistent with basal stem rot symptoms observed in the original outbreak (Figure 4B), whereas controls remained symptomless (Figure 4A). Monilochaetes sp. was consistently reisolated from symptomatic plants, fulfilling Koch's postulates. The assay was repeated three times with similar results. Monilochaetes sp. was confirmed as the causal agent of basal stem rot of water spinach. Although most closely related to M. dimorphospora, sequence divergence and limited reference data precluded species-level identification. Monilochaetes infuscans, the causal agent of sweet potato scurf, has been reported on sweet potato (Ipomoea batatas) in Florida (Alfieri 1984; Réblová et al. 2011; Dewayani et al. 2021). To our knowledge, this is the first report of Monilochaetes sp. causing basal stem rot on water spinach (Ipomoea aquatica) in the United States, expanding the known host range of the genus. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.