First report of rust caused by Gymnosporangium in Iceland.

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Sorbus is a genus of trees and shrubs in the Rosaceae commonly known as rowan and mountain-ash. They are usually found in temperate regions of the Northern Hemisphere and cultivated as ornamental trees for parks and gardens. In September 2023, infection by a rust was observed on a single Sorbus aucuparia tree in Sólbrekkuskógur, Reykjanesbær (64.046645, -22.707276; ~13 m) in Iceland. Infected leaves were collected from this single cultivated tree at an outdoor recreation area in a natural wooded location, with a 2% disease severity. Sori were infrequent, scattered, embedded within circular yellow lesions on leaf margins. On average, one sorus was observed per leaf and only 2% of leaves were infected. Spermogonia epiphyllous, punctate and aggregated, pale yellow to black. Hypophyllous aecia roestelioid with cornute peridium rupturing at apex with peridial cells rhomboidal, aeciospores yellowish brown globoid 17.67-25.17 x 17.20-21.94 µm, walls 1.22-2.28 µm thick (n = 20). The features of this rust and dimensions of spores are consistent with descriptions of Gymnosporangium cornutum (Arthur 1909, Kern 1911). To confirm the identity (specimen MCA9732), a ~620 bp region of the 28S subunit of the ribosomal DNA repeat was sequenced using primers Rust2inv and LR6 following published protocols (Aime 2006). The sequence (GenBank PP413765) shared 100% (649/649 bp) identity with a sequence deposited as Gymnosporangium cornutum (KY764066, BPI910184; J. E. Demers, M. K. Romberg, and L. A. Castlebury, unpublished data) from S. americana and 100% (620/620 bp) identity with G. cornutum (PURN11049) on Sorbus sp. from Canada when blasted against the RustHUBB database (Kaishian et al. 2024). The specimen has been deposited in the Arthur Fungarium at Purdue University as PURN24233. Disease on Sorbus sp. caused by G. cornutum has been reported in various countries in Africa, Asia, North America, and Europe (Kern 1911). To our knowledge, this is the first report of the occurrence of this genus in Iceland from any host. Gymnosporangium cornutum alternates on Juniperus species. In Iceland, J. communis (sect. Oxycedrus) seems to be the only naturally occurring Juniperus species but it is an alternate host for G. cornutum. The presence of the primary and alternative hosts in Iceland and the ability of Gymnosporangium spp. to produce systemic infections in Juniperus spp., represents the potential for reinfection of Sorbus every year, resulting in potential impacts on both host species. With J. communis being the only Juniperus spp. in natural habitats in Iceland, the presence of this rust represents a potential ecological disruption, as repeated infections may reduce host vitality and predispose the host to winter injury and attack from opportunistic pathogens or insects.