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Malva sylvestris (Malvaceae), known as common mallow, is native to Europe, western Asia, and northern Africa. It was intentionally introduced to Korea as an ornamental plant in the early 20th century, and has become partly naturalized in several areas including the woods (Jung et al. 2017). Among nine microcylic Puccinia species attacking the Malvaceae plants, three species of P. heterospora, P. malvacearum, and P. modiolae have been reported on M. sylvestris (Classen et al. 2000, Colenso 1885, McKenzie 1998 and Melo et al. 2012). Only P. modiolae has been found on Alcea rosea and M. verticillata, and not M. sylvestris in Korea (Lee et al. 2022; Ryu et al. 2022). In August 2022, rust disease symptoms of a Puccinia fungus were observed on some overgrown seedlings of M. sylvestris, which were neglected in containers after sales at a wholesale nursery (36°50'19.8″N, 128°55'28.7″E) in Bonghwa, Korea. Typical rust spots were observed around 60% (on 111 seedlings of the 186 seedlings of M. sylvestris). The brown spots were produced on round chlorotic haloes on the adaxial leaf surface, and brown to dark brown pustules on the abaxial. Subepidermal spermogonia on the adaxial, were obovoid, and 112.1-160.0 × 88.7-149.3 μm in size. Telia were golden-brown to dark brown, round, mostly grouped, and 0.30-0.72 mm in diameter, and mainly hypophyllus. Fusoid teliospores were two-celled, rarely one- or three-celled, 36.2-92.3 × 10.6-19.3 μm in size, with many anomalies appearing notched at apex; wall was yellowish or almost colorless, smooth, 1.0-2.6 μm thick at the sides, and up to 6.8 μm thick at the apex; pedicel was hyaline, thick wall, persistent, and (39.3-)60.4-154.6(-189.9) μm long. Based on these morphological features together with the results of the phylogenetic analyses (e-Xtra 2) using internal transcribed spacer (ITS) and partial large subunit (LSU) sequences according to the method described by Ryu et al. (2022), the fungus was identified as an autoecious P. modiolae, recently reported on M. verticillate and A. rosea in Korea (Lee et al. 2022; Ryu et al. 2022). A representative sample was deposited in the Animal and Plant Quarantine Agency Herbarium (PQK220818). Pathogenicity tests were done using three host plants: M. sylvestris, M. verticillate and A. rosea. Three to four leaf discs with basidiospore-bearing telia were placed on the upper surfaces of healthy young leaves of the seedlings. Three replicates of each host plant set including an untreated control were tested. The plants were kept in an isolated glass house. At ten to twelve days after inoculation, typical telial spots of P. modiolae were recovered, but not in the control plants, showing all three tested species were highly susceptible (e-Xtra 1). The ITS and LSU sequences obtained from the genomic DNAs of each newly recovered rust spot were consistent with that of the inoculum (accession no. OQ542745). The previous A. rosea isolate (OP369290 by Ryu et al. 2022) also showed the pathogenesis on M. sylvestris and M. verticillata by the same tests, mentioned above (e-Xtra 1). To date, only one collection of P. modiolae on M. sylvestris has been reported in Louisiana, the United States (Aime and Abbasi 2018). The results of this study show that P. modiolae is firstly confirmed as the causal rust fungus of M. sylvestris and the same causal agent of M. verticillate and A. rosea rust disease, recently reported in Korea.
Mobasher et al. (Sat,) studied this question.