First report of <i>Austropuccinia psidii</i> on <i>Syzygium buxifolium</i> grown as indoor bonsai in Europe

In May 2024, bright yellow pustules were observed on the stems and leaves of a boxleaf eugenia (Syzygium buxifolium) plant sold as an indoor bonsai at a Swiss branch of a large European garden retailer. Four additional branches of the same garden retailer were thereafter visited, and one diseased S. buxifolium plant was collected from each. The total number of diseased plants at each branch was not systematically assessed. Early symptoms included red-purple spots on leaves and shoots, progressing to bright yellow uredinia (Figure 1). Older lesions turned brown with sub-angular margins, causing petiole and shoot deformation, occasionally leading to leaf and twig dieback. Microscopy confirmed the presence of pyriform to spherical, echinate urediniospores (16–24 × 13–20 µm; Figure 2), no teliospores were detected. The host and morphological information suggested that the rust was caused by Austropuccinia psidii (Beenken, 2017). To verify the identification, DNA was extracted from all diseased plants and the ITS2-LSU region was sequenced using rust-specific primers Rust2inv, LRust1R, LRust2 and LR6 (Beenken et al., 2012). The sequences obtained were identical to GenBank Accession No. OR082910 (A. psidii from Syzygium jambos). Four of the five plants sampled were confirmed to be infected with A. psidii. Using four microsatellite markers (PpSSR012, PpSSR014, PpSSR018, PpSSR087), the strains were further identified as belonging to the pandemic biotype associated with the emergence of myrtle rust on various hosts across different geographic regions (Stewart et al., 2018). The sequences were submitted to GenBank (PQ039749–PQ039752) and preserved plant material was deposited in ETH Zurich (ZTMyc66432). Pathogenicity tests were conducted on S. buxifolium and common myrtle (Myrtus communis), a susceptible host native to Europe (Paap et al., 2023). A spore suspension (4 × 10⁶ spores/ml with 0.05% Tween 20) was sprayed onto plants. After five days, brown discolouration and lesions appeared on both hosts, followed by uredinia development 11 days post-inoculation (Figure 3), completing the disease cycle. The identity of the pathogen as A. psidii was confirmed by microscopy and DNA sequencing. The pathogenicity tests included a control treatment for both hosts where plants were sprayed with 0.05% Tween 20 solution without spores, and no symptoms developed on these plants. Upon detection of A. psidii, all infected plants were removed from sale and destroyed. Details of how long the plants were on sale or how many were sold are unavailable. The risk of an outbreak in Switzerland is considered low due to unsuitable climatic conditions and the absence of native host species. However, the retailer's European wide distribution network poses a phytosanitary risk, particularly in regions like Italy where susceptible native species and favourable climates exist. This is the first report of A. psidii causing rust disease both in Europe and on S. buxifolium. The finding emphasizes the phytosanitary risks associated with the international plant trade. Although A. psidii was removed from the European and Mediterranean Plant Protection Organization (EPPO) Alert List in 2003 (EPPO, 2003), it remains a significant concern in certain areas. For example, the pathogen is listed on the UK Plant Health Risk Register and subject to statutory action to prevent its establishment and spread. The detection of A. psidii in retail is concerning, especially given its potential impact on species such as common myrtle, native to the European Maquis shrubland, and the economically important Eucalyptus genus. Ornamental Eucalyptus plants were located near the infected sporulating bonsai plants in the garden centre, though their susceptibility to A. psidii was not assessed. The infected bonsai plants were traced to a nursery in the Netherlands, as indicated by the plant passport registration number. The introduction of A. psidii via the plant trade could serve as reservoir for the pathogen and be highly detrimental in non-native habitats. The expanded host range and recent interception within the EPPO region have led the Panel on Phytosanitary Measures to re-add A. psidii to the EPPO Alert List (EPPO, 2024). We thank Antonello Speroni and Sven Orbach. The laboratory testing was supported by the Swiss Federal Office for the Environment (FOEN).