Puccinia psidii first appears as chlorotic specks that develop into pustules containing uredinia which produce yellow masses of urediniospores. Pustules can coalesce and parts of the plant can be completely covered with pustules. (Liberato et al. 2007). Uredinia occur mostly on the underside of leaves, on stems and on flowers and fruit. They appear as pale yellow to yellow-orange dusty spots 0.1-0.5mm in diameter, in groups on brownish or blackish spots up to 5mm. Urediniospores are globose and ellipsoid to ovoid in shape, measuring 19-27 x 15-26 µm. The spore cell walls echinulate, hyaline to yellowish in colour and 1.5-2.5 µm thick with germ pores obscure (Hernández 2006).

Teliospores appear in the uredinia or telia (Liberato et al. 2007). Teliospores are club shaped, ellipsoid to oblong measuring 30-48 x 17-22µm.They are rounded at the apex, narrow below and slightly constricted at the septum. The cell walls are pale yellow in colour, smooth and measure 1.5-2.5 µm thick at sides and 2-4 µm at apex (Hernández 2006). Teliospores are bicellular, and both cells can germinate to form basidia. Basidia are 40 to 70 µm in length and only 0.1% produce basidiospores, which are borne on sterigmata (Ferreira 1989 in Coutinho et al. 1998). It is not known if the basidiospores are uni- or bi- nucleate (Simpson et al. 2006). Aeciospores are morphologically identical to urediniospores (Liberato et al. 2007).

Symptoms on susceptible hosts appear as brown to grey lesions with masses of yellow or orange-yellow urediniospores on young, actively growing leaves, shoots, fruits and sepals. Lesions may have sori containing dark brown teliospores. Purpling of leaf and shoot margins occur on older lesions in some host species. Severe disease in young trees may kill shoot tips, causing loss of leaders and a bushy growth form to develop. In eucalypts prolific branching and galling is a symptom of previous rust infection (Glen et al. 2007).

Please see PaDIL (Pests and Diseases Image Library) Species Content Page Fungi: Eucalyptus rust for high quality diagnostic and overview images.

The anamorphic (asexual) stage/morph of Puccinia psidii is Uredo psidii. A detailed description is available in Simpson et al. (2006).

Several races or biotypes of P. psidii are known to exist, which differ in their host specificity, environmental tolerances, characteristics of sporulation and spore survival, and virulence. However very little is known of these specialized forms (Glen et al 2007).

Puccinia psidii has variously been reported as an autoecious species, meaning that it uses only one host for its lifecycle (Loope and La Rosa 2008) and heteroecious with an undetermined alternate aecial host (Simpson et al. 2006). It has a macrocyclic life cycle, producing aecia, uredinia, telia and basidia which produce aeciospores, urediniospores, teliospores and basidiospores respectively. Spermatia have not yet been found, but due to the reported occurrence of aecia this stage must exist (Coutinho et al. 1998). The asexual dikaryotic urediniospores are the most prevalent stage in the life cycle. Teliospores and basidiospores are rare in nature, although teliospores are more frequent on some host species. Telia are usually produced under hot weather or on mature plant tissue. Aeciospores have not been observed or recognised in nature as they are morphologically identical to urediniospores (Glen et al. 2007). Old et al. (2003) report that \"Disease spread results from airborne urediniospores alighting on leaf surfaces and germinating during darkness and conditions of leaf wetness at temperatures of 15–25°C. Rust lesions then become visible 2–4 days later. Penetration of mature leaves is generally unsuccessful and this is consistent with the observation that, on Eucalyptus, trees less than two years old are susceptible to disease. Coppice shoots of older trees are, however, vulnerable to infection due to their juvenile foliage\". Following penetration on a susceptible plant, the fungus grows intercellularly through the host tissue and produces haustoria within host cells. Germ tubes of basidiospores and aeciospores have been reported to penetrate the host directly (Coutinho et al. 1998). In incompatible reactions, caused by non-host species or resistant varieties smaller atypical pustules are formed on leaves (Ferreira and Silva 1982 in Coutinho et al. 1998). Hypersensitive reactions, where death of a few cells at the site of penetration have been observed in some non-compatible reactions (Coelho 1988; Ferreira 1989 in Coutinho et al. 1998).

A diagram detailing the lifecycle of P. psidii is available in Glen et al. (2007).

In Florida, the pathogen has been considered as a biological control agent for the invasive Australian plant, Melaleuca quinquenervia (Rayachhetry et al. 2001).

Puccinia psidii has a remarkably wide host range for a rust pathogen (Tommerup et al. 2003). Over 70 species in 20 genera of the Myrtaceae family are hosts for P. psidii (Simpson et al. 2006), and all genera in this family are potentially susceptible (Old et al. 2003). It was recently discovered that the fungus is able to infect a species in the Heteropixidaceae family (Alfenas et al. 2005). Evidence of host specialisation exists within the pathogen, so isolates from one host genus may or may not infect other genera within Myrtaceae (Old et al. 2003).

Asexual: The asexual dikaryotic urediniospores are the most prevalent stage in the life cycle (Coutinho et al. 1998).
Sexual: It is not known if P. psidii is homo- or hetero-thallic. It is possible that P. psidii is apomictic and the first spores produced from basidiospore infections are already urediniospores (Simpson et al. 2006).

Puccinia psidii affects members of the Myrtaceae family. According to Liberato (2007) \"The fungus attacks young tissues of new leaves, fruits, flowers, shoots and succulent twigs. The first symptoms are chlorotic specks which become, after a few days, pustules containing uredia producing yellow masses of spores. Pustules can coalesce and parts of the plant can be completely covered with pustules. After about 2-3 weeks, pustules dry and become necrotic. The disease can cause deformation of leaves, heavy defoliation of branches, dieback, stunted growth and even death. On fruits of guava (Psidium guava, Psidium spp.), Ruberry (Myrcia spp., Myrciaria spp.) and other native American hosts in the Myrtaceae, the lesions occur mostly on buds and young fruits that eventually rot as the rust matures.\"

Tommerup et al. (2003) report that “The disease is particularly severe on susceptible eucalypt seedlings, cuttings, juvenile trees, or coppice from stumps or damaged mature trees. Heavy infection of juvenile eucalypt shoots causes plants to be stunted and multi-branched, reducing their potential to develop into marketable trees”. There is wide variety in susceptibility to P. psidii among young eucalypt trees. Usually around 10-20% of a stand is highly susceptible, causing gross malformations and death. The pathogen is particularly virulent on species with no evolutionary history of exposure (naïve hosts) (Loope and La Rosa 2008).

P. psidii has already had devastating impacts on some industries. Oil refineries based on Pimenta dioica in Jamaica were forced to close two years after P. psidii was first recorded on the island (Maclachlan 1938 in Booth et al. 2000).

There is also potential for this pathogen to cause damage in natural ecosystem. The family Myrtaceae consists of approximately 3600 species in 155 genera distributed across Australia, South-east Asia, New Caledonia and other Pacific Islands, South and Central America and southern Africa (Tommerup et al. 2003). In many of these countries susceptible trees are significant components of communities. For example in Australia many ecosystems are dominated by Eucalyptus (Booth et al. 2000), and in Hawai'I the ohia (Metrosideros polymorpha) is the dominant tree in most of Hawai'I's forests. Outbreaks of P. psidii which affect these dominant trees could result in significant changes to the structure, composition, and potentially, the function, of forests on a landscape level. This would likely have impacts on biodiversity of other flora and fauna in these ecosystems (Loope and La Rosa 2008).

Preventative measures: Quarantine restrictions are the most effective means of preventing introductions of potentially virulent strains of Puccinia psidii (Loope and La Rosa 2008). In 2007 the Hawai'ian state Board of Agriculture passed an order which banned plant products from California, Florida and South America that could be disease hosts. The ban includes any plants of the Myrtaceae family (Tanji 2007).

Detection and Prediction: Early, rapid and accurate pathogen diagnosis is critical for management of high-quarantine-risk pathogens such as P. psidii. Langrell et al. (2008) developed a species-specific, nested polymerase chain reaction (PCR)-based detection assay for P. psidii. They report that “The assay reliably, accurately and sensitively detected the rust from naturally infected, geographically widespread eucalypt and fruit tree plantation and nursery species from diverse tissues types including symptomless or cryptically contaminated plants or plant tissue.” This assay provides an effective biosecurity tool for rapid responses to threats or incursions of P. psidii (Langrell et al. 2008).

Booth et al. (2000) modeled disease-prone regions using climate interpolation and disease-hazard mapping methods. They showed that areas highly prone to the rust occur across tropical and subtropical regions globally. These areas are likely to coincide with major eucalypt- and melaleuca-oil production areas (Booth et al. 2000).

Chemical: Fungicide use is most appropriate in nurseries and clonal hedge plantings, as it is not cost effective for large areas. Fungicides used include the protectant fungicide mancozeb and the systemic fungicides triamenol and triforine. In most cases, however, resistant trees are chosen over susceptible varieties, eliminating the need for fungicides (Old et al. 2003).

Biological: There are a number of biological control agents with potential for management of P. psidii. Bacillus subtilis was found to be effective in reducing in vitro germination of P. psidii, although there are no field experiments to confirm this and it may be difficult to reproduce in vivo (van Toor et al. 2005 in Glen et al. 2007). However these initial results are promising and deserve further investigation.

Other biological agents with potential include Fusarium decemcellulare, a hyper-parasite of P. psidii (Amorium et al. 1993 in Glen et al. 2007), and other fungi which co-occur in rust pustules. Strains of rhizobacteria (Pseudomonas aeruginosa) which have been demonstrated to induce systemic resistance in some trees may be developed into tools for managing disease (Glen et al. 2007). Glen et al. (2007) however, state that “Much further work is also necessary before any fungus or bacterium can be recommended as a biological control agent.”

Breeding Resistance: According to Glen et al. (2007) “Resistance represents the most economically viable approach to controlling P. psidii where it has become established.” In Brazil guava cultivars have been evaluated for resistance, and resistant progeny selected for breeding (Ribeiro and Pommer 2004). Also elite hybrid clones of Eucalyptus grandis and E. grandis x Eucalyptus rophylla are widely used by the cellulose and paper industry in Brazil as E. grandis is highly susceptible to P. psidii (Junghans et al. 2004 in Glen et al. 2007). Selection of resistant species and clones has been a high research priority in Brazil (Xavier et al. 2001 in Glen et al. 2007). A single dominant allele conferring rust resistance has been identified and mapped in E. grandis (Junghans 2000; Junghans et al. 2003 in Tommerup et al. 2003), which will further assist selection of resistant offspring.