Background and aims – Struthanthus is a neotropical genus with 60 to 70 species, distributed from northern Mexico to Argentina. Eighteen species are reported in Mexico. During fieldwork to investigate Struthanthus in Mexico, a new species was collected in the state of Guerrero. We present a full description, distribution map, illustrations, and photographs of an unusual new Struthanthus species endemic to Guerrero, Mexico.

Material and methods – This study is based on fieldwork, literature revision, and examination of herbarium material. Collected specimens were prepared for storage in the herbarium, analysed, measured, described, and photographed. Preliminary conservation assessment is based on spatial analyses following IUCN guidelines and criteria.

Key results – We propose a new species of Struthanthus for Guerrero, Mexico, S. longipetiolatus. This species is only known from cloud forests, oak-pine forests, and oak forests in the province of Sierra Madre del Sur, in the state of Guerrero. A distribution map and an identification key are provided for S. longipetiolatus and its morphologically similar congeners in the region.

Conclusion – The newly described species increases the number of Struthanthus species in Mexico to 19 and the number of endemic species to nine. This discovery underlines the need for continued botanical inventories and research on parasitic plants.

endemism, hemiparasite, mistletoe, taxonomy

Struthanthus Mart. is a complex genus of Loranthaceae because it shares morphological characters with other genera of small flowers, such as Cladocolea Tiegh. (Kuijt 1987, 1991b), Panamanthus Kuijt (Kuijt 1991a), Passovia H.Karst. (Kuijt 2011; Caires et al. 2021), Peristethium Tiegh. (Kuijt 2012; Caires et al. 2014; Caires and Azevedo 2015), Phthirusa Mart. (Kuijt 2009), and Pusillanthus Kuijt (Caires et al. 2012). Struthanthus is closely related to Cladocolea and Peristethium, genera recognized as polyphyletic (Kuijt 1975, 1981, 2012; Kuijt and Hansen 2015; Galván-González et al. 2024). Struthanthus is a neotropical genus distributed from northern Mexico to Argentina, comprising 60–70 species (Caires and Dettke 2020). About 18 species of Struthanthus have been reported from Mexico (Galván-González et al. 2024; Maldonado-Borja et al. 2024). The exact number of species within Struthanthus remains uncertain due to the lack of a comprehensive taxonomic treatment (Kuijt 1964, 2003b, 2012, 2016; Dettke and Waechter 2012; Caires and Azevedo 2015; Robles et al. 2016; Maldonado-Borja et al 2023). According to Martínez-Ambriz et al. (2017), four species of Struthanthus are distributed in Guerrero: S. crassipes (Oliv.) Eichler, S. interruptus (Kunth) G.Don, S. quercicola (Schltdl. & Cham.) G.Don, and S. ramiro-cruzii Martínez-Ambr. & Sor.-Benítez. Recently, Cladocolea racemosa Kuijt was transferred to Struthanthus as S. racemosus (Kuijt) Galv.-González (Galván-González et al. 2024). Here, we describe a new species of Struthanthus from Guerrero, Mexico that shares morphological affinities with S. ibe-dzi Mald.-Borja & Cerros, S. racemosus, and S. ramiro-cruzii.

The description is based on our collections from Sierra Madre del Sur (SMS) in Guerrero and herbarium specimens from ENCB, HUMO, MEXU, and XAL (acronyms follow Thiers 2024). Specimens were collected, pressed, and dried, with three to five duplicates. Vouchers were deposited in HUMO and duplicates in HUAP, MEXU, and UAMIZ. Vegetative and reproductive structures were carefully studied under a stereoscopic microscope. Measurements and photographs of the floral structures were taken using fresh or rehydrated material. Morphological descriptions were based on Kuijt’s terminology (Kuijt 1987, 2003a, 2003b). The map was generated with QGIS v.3.16 (QGIS Development Team 2020) using data from our fieldwork and herbarium labels. The conservation status assessment of the new species follows the IUCN Red List categories and criteria (IUCN 2022). The extent of occurrence (EOO) and area of occupancy (AOO), using 2 × 2 km grid cells (4 km²), were estimated using the Geospatial Conservation Assessment Tool (GeoCAT; Bachman et al. 2011).

Struthanthus longipetiolatus morphologically resembles S. ibe-dzi, S. racemosus, and S. ramiro-cruzii because the four species have inflorescences in racemes. Additionally, unlike other Struthanthus species, they have stamens fully attached to the petal and lack trichomes at the base of the petal’s insertion with the filament. The four species are distributed in the Sierra Madre del Sur, with S. ibe-dzi being endemic to the state of Oaxaca. In contrast, S. longipetiolatus, S. racemosus, and S. ramiro-cruzii are found in the state of Guerrero. Like S. ramiro-cruzii and S. ibe-dzi, S. longipetiolatus has triads with pedicellate lateral flowers and a sessile central one. Struthanthus racemosus differs from S. longipetiolatus in having monads instead of triads in addition to petioles 0.60–0.90 mm long. Additionally, S. longipetiolatus and S. ramiro-cruzii are found in the same locality, but they differ mainly in the leaf shape and pedicel length. Moreover, S. ramiro-cruzii has leaves ovate to lanceolate, cordate when mature, with a petiole 0.5–1.0 cm long while S. longipetiolatus has leaves lanceolate, rarely ovate with apex acute to long acuminate, mucronate, and a base acute to obtuse with a slender petiole up to 3.20 cm long. Another notable difference is the stem colour, which ranges from olive green to pink-reddish, and extends to leaves, flowers peduncles, petals, and even the styles. Plant pigments are involved in various functions including photosynthesis, for example, flowers from calyx to pistil may accumulate pigments implicated in pollination (Miller et al. 2011). According to Heide-JØrgensen (2008), the red colour is due to anthocyanins in the vacuoles of the cells. In S. longipetiolatus, the change of colour from olive green to pink-reddish in the floral parts, mainly the styles, could be related to the fertilization of the flower. According to Miller et al. (2011), floral pigments are modified during flower development, for example, at emergence and after pollination. This change could direct pollinators to unpollinated flowers. Struthanthus longipetiolatus has petioles arranged in a pendant manner, unlike other species of Struthanthus in Mexico, which have ascending petioles.

Guerrero ranks as the 4^th state in Mexico in terms of biological diversity (Botello et al. 2015) and the 5^th in plant diversity, with 237 endemic species (Villaseñor 2016). Additionally, 5.4% of its area is designated as Federal Protected Natural Areas (ANPs; DOF 2023; CONANP 2024). The loss of natural vegetation is 2.46% per year (SEMARNAT 2015). Each year, 0.5–0.7% of coniferous forests and 2.4–2.7% of tropical forests are lost. In addition, forest fragmentation in the country is estimated between 22 and 24%, with overgrazing at an annual rate of 23.7–36.3% (SEMARNAT 2015). This makes S. longipetiolatus a vulnerable species due to habitat quality decline. Despite the efforts by the Facultad de Ciencias at UNAM to document the Flora of Guerrero (Diego-Pérez and Fonseca 1994–2024), the state has historically faced social challenges such as poverty, natural disasters, violence (including drug trafficking and forced displacements). These issues have significantly hindered the exploration and knowledge of its biological diversity. Thus, it is crucial to understand the challenges the state faces and recognize the importance of protecting and describing the flora and fauna of Guerrero.

We are grateful to Emmanuel Martínez-Ambriz, an anonymous reviewer, the subject editor Luiza Teixeira-Costa, and the editor in chief Brecht Verstraete for their comments and suggestions to improve this manuscript. This work is part of the MSc thesis of the first author and is financially supported by the National Council of Science and Technology (CONAHCYT) through grant 286670. We thank the curators from the herbaria for allowing access to their collections, particularly to ENCB, HUMO, MEXU, UAGC, and XAL for specimen loans. Special thanks to the Secretaría de Medio Ambiente y Recursos Naturales for granting a collecting permit (SGPA/DGGFS/712/1132/1809/N1-0081/04/18). A sincere and deep gratitude to the brothers Germán^†, Hipólito, and Jerónimo Maldonado Marino and their respective families, for their support and company during the fieldwork. Also, our gratitude to the family Solano Moreno from Llanos de Tepoxtepec, especially to the fearless young Santiago. We also thank Dr J.C. Caspeta-Mandujano for his help during fieldwork, Eduardo Santaella for the illustrations, and Elizabeth Skendzic for the final review of the manuscript.