First Record of <i>Puccinia duthiei</i> on <i>Bothriochloa bladhii</i> and <i>B. ischaemum</i> in the USA

In March and April 2025, diseased plants of Bothriochloa bladhii, known as purple plume grass and Australian bluestem, Plant Industry Herbarium Gainesville (PIHG) 19403 (N26.063141:W80.139230), 19404 (N26.123970:W80.1722360) and B. ischaemum, known as yellow bluestem and King Ranch bluestem 19405 (N26.055690:W80.253130), were collected in urban areas in the cities of Dania Beach, Davie and Fort Lauderdale, all located in Broward County, Florida, USA. Microscopic examination revealed abundant orange to dark brown, erumpent and pulverulent sori, measuring c. 0.5 mm in length. These were particularly prevalent on older leaves, covering up to 50% of the leaf surface. The sori were observed to be subepidermal and paraphysate, featuring capitate and clavate paraphyses that were yellowish-brown, smooth-walled, and up to 95 µm long and 23 µm at their widest point. Urediniospores were golden to cinnamon-brown, echinulate, ovate to subglobose, measuring 22–34 × 21–26 µm, with 3–5 equatorial to subequatorial germinal pores and thin walls. Telia were produced after the uredinia. Two-celled teliospores were detected, chestnut-brown in colour, slightly constricted at the septum, with smooth, thick walls, measuring 36–48 × 20–30 µm, with hyaline to yellowish pedicel centrally or slightly eccentric, persistent, wider at the base of the teliospore, and longer than 110 × 7–11 µm wide (Figure 1). These morphological characteristics are consistent with the description of Puccinia duthiei (Pucciniales, Basidiomycota), as detailed by Narasimhan and Thirumalachar (1968). We conducted molecular characterisation of the fungal pathogen by PCR amplification and Sanger sequencing of the cytochrome oxidase subunit 3 (CO3, c. 670 bp, GenBank Accession Nos. PV871698-PV871670) and the large subunit of the ribosomal RNA gene (LSU, c. 890 bp, PV871591-PV871593) (Aime et al. 2006; Otálora and Berndt 2018). Sequence comparison revealed over 98.6% sequence identity for the CO3 gene against P. melanocephala (MW139660) and for the LSU gene against P. duthiei (KX999875) (Chen et al. 2015; Kaishain et al. 2024). Our phylogenetic analysis further corroborated these findings, aligning with both the morphological and molecular identifications (Figure 2). P. duthiei is a heteroecious rust with Barleria cuspidata (Laminales, Acanthaceae) as the alternate host for the aecial stage (Narasimhan and Thirumalachar 1968). However, P. duthiei has not been detected yet in any of the species of Barleria in Florida. This rust has been documented on several species of Bothriochloa as well as other genera of tall grasses mainly across Asia, and has also been reported in regions including Australia, the Caribbean, Mexico and South America (Farr and Rossman 2018). Based on our comprehensive literature review, this study represents the first report of P. duthiei in the continental USA, specifically Florida, infecting B. bladhii and B. ischaemum. As with most rust pathogens, P. duthiei poses a potential threat to economically important grasses raising concerns for agricultural and ecological systems. Therefore, pest risk assessments and ongoing surveillance are crucial to monitor its spread and potential impact on both native flora and cultivated grasses in the region. Further research into the potential for evaluating P. duthiei as a biocontrol agent against the invasive B. ischaemum is warranted.