Hi welcome to Taiga Bonzai, in this article we continue our research on new plant diseases be they mutational strains from existing ones or completely new.

Introduction – Syzygium buxifolium is one of the most sensational evergreen flowering easy care indoor bonsai tree variety and is one of the more hardy species. It is noted for its sensational soft cream flowers with long central stamens. The plant has other names including, Fish-scale bush, Eugenia microphylla, Eugenia sinensis, Syzygium somae, Eugenia pyxophylla, Syllysium buxifolium, Box-leaved syzygium.

However, contrary to popular belief this species is not as hardy as one is lead to believe. Because it is prone to attack from the fungal pathogen Austropuccinia psidii, which is found all over the globe. A rust fungus with a wide and expanding host range within the Myrtaceae, with over 440 host species currently known. Like many rusts, urediniospores of A. psidii can be wind-dispersed over long distances.

It has spread quickly once established in new countries, including Jamaica (MacLachlan, 1938), Hawaii (Uchida and Loope, 2009), Australia (Carnegie and Cooper, 2011; Pegg et al., 2014) and New Caledonia (DAVAR Nouvelle-Calédonie, 2014).

In May 2024, bright yellow pustules were observed on the stems and leaves of a box leaf 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.

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).

Early symptoms included red-purple spots on leaves and shoots, progressing to bright yellow uredinia. 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; 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). According to (Langrell et al., 2003) “Viable spores have been detected on clothing and personal effects following visits to rust-affected plantations, and this is a viable pathway for dispersal.”

Furthermore, there are several instances of (accidental) long-distance movement of A. psidii on diseased plants, both within and between continents. (Loope et al., 2007; Kawanishi et al., 2009; Carnegie and Cooper, 2011; Zambino and Nolan, 2012).

Under sub-optimal conditions, the rust can remain un-symptomatic within plants for more than a month. (Carnegie and Lidbetter, 2012) This combination of wide host range and ease of long-distance dispersal make A. psidii a successful invasive pathogen.

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 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.

Severe impact on a range of Myrtaceae has been recorded in amenity plantings, commercial plantations and the native environment. A. psidii was first identified as an invasive pathogen in the 1930s when it caused extensive damage to allspice (Pimenta dioica) plantations in Jamaica (Smith, 1935; MacLachlan, 1938).

A. psidii has been identified as a quarantine risk for some time in many countries including Australia (Australian Quarantine Service, 1985 Grgurinovic et al., 2006), South Africa (Grgurinovic et al., 1998) and New Zealand (Kriticos and Leriche, 2008).

The scientists who did the major research on this article that was first published 17 December 2024 are as follows. B. Ruffner, L. Beenken, Q. Kupper, J. Mittelstrass, P. Schuler, J. E. Stewart, J. R. Ibarra Caballero, R. Winiger, S. Prospero. https://doi.org/10.1002/ndr2.70011

In the next article on this subject we continue our research be they mutational strains from existing ones or completely new, because this information is up to date and important for horticulturists. Until next time, BW, Nik.