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|a The dwarf hawthorn Crataegus uniflora Münchh. (Rosaceae, Maloideae) is a small deciduous tree species native to the central and eastern US and south into northern Mexico. Dwarf hawthorn is drought tolerant and commonly found in disturbed areas (e.g., hedges and roadsides). In May 2021, we observed several individuals of dwarf hawthorn growing on the border of an empty field in the Natural Area Teaching Laboratory at The University of Florida main campus in Gainesville, Alachua County, Florida, USA (N29.633382, W82.368350) that were severely infected by fruit galls with visible, whitish aecia (e-Xtra Fig.1). The affected fruit were collected and transported to the Florida Department of Agriculture and Consumer Services - Division of Plant Industry headquarters in Gainesville for identification (FDACS-DPI, 2021-107788). The conspicuous rust fungus, occurring on the fruit (fructicolous), consisted of tubular aecia (roestelioid), 4 - 5 mm in length × 0.5 mm in diameter, with whitish peridia containing bright orange spores in masses. Aeciospores were semigloboid to globoid, some with an angular side, with bright orange contents, 26 - 31 µm in diameter (n= 20). The wall was densely verrucose, hyaline, 3 - 4 µm wide. Side and wall ornamentation are considered diagnostic features (EPPO Bulletin, 2006). Peridial cells of the aecia were hyaline, angular (pentagonal to hexagonal) to irregular, with a thin, convoluted wall, 41- 57 × 30 - 35 µm (n =10). Aeciospores were detected on blackish, mummified berries five months after the initial collection and aecium disappearance. This persistence demonstrates one of the adaptations allowing the pathogen to remain in a given location (e-Xtra Fig.1). The morphological characteristics are consistent with those of Gymnosporangium clavipes Cooke & Peck (Gymnosporangiaceace, Pucciniaceae, Pucciniomycotina) described by Kern (1973). A voucher was deposited in the DPI Herbarium (PIHG, specimen number 15618). The morphological identification was confirmed by molecular identification: following DNA extraction (DNeasy Plant Pro extraction kit, Qiagen Corporation, Hilden, Germany), we amplified a fragment of the internal transcribed spacer (ITS) and the large subunit (LSU) via PCR using the primer pairs Rust2inv/LR6 and Rust28S/LR5 (OK337508) (Aime, 2006); amplicons were then Sanger sequenced. NCBI megaBlast searches (Chen et al. 2015) of the resulting fungal sequences revealed high identity (ITS and LSU) to two G. clavipes vouchers: NYBG461394 (99.86%, Genbank accession no. MN605691) sequenced in the latest publication addressing species delimitation in Gymnosporangium (Zhao et al., 2020), and PPST 2020-104160 (99.72%, GenBank MW148514): the first report of this rust occurring on Crataegus marshallii Eggleston (McVay et al., 2021), also recently found in Gainesville, Florida. Phylogenetic analyses were carried out in the phylogenetic package RAxMLv8.0.0 (Stamatakis, 2014) (e-Xtra Fig. 2) further supports placement of 2021-107788 within G. clavipes. The heteroecious nature of this rust fungus precludes Koch's postulates. Based on exhaustive reviews of collection indices and literature, a specimen of G. clavipes on C. uniflora exists at the U.S. National Fungus collections (BPI 117783A) collected in Newfield, New Jersey in 1888 (Farr & Rossman, 2022); this rust fungus has a host range of at least 18 other species of Crataegus (Farr and Rossman, 2022; McVay et al., 2021; Zhao et al., 2020). This report represents the first published record of G. clavipes on dwarf hawthorn, and the first report in Florida
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