Detailed descriptions have been supplied by Walker (1983), Simpson et al. (2006) and Pegg et al. (2014) and in reviews by Coutinho et al. (1998) and Glen et al. (2007). Lesions mainly appear on young, actively growing leaves and shoots, but also on flowers and fruits. Often the first sign of infection is chlorotic flecks on leaves and shoots, followed by the production of masses of bright yellow urediniospores;more rarely yellow-brown teliospores are produced, often intermingled with urediniospores. Lesions often turn red-purple then grey with age, and often have a purple or dark brown margin. Lesions tend to be angular in shape, extending through the leaf, and more often coalescing. Uredinia are 0.1-0.5 mm diam., amphigenous, yellowish (but fade to pale tan when old), more common and larger on the abaxial surface, subepidermal becoming erumpent and up to 500 μm. Urediniospores vary from globose, ellipsoidal to ovoid and obpyriform, are yellowish, 14-27 x 14-29 μm, finely echinulate, with or without a tonsure;germ pores have not been observed. Telia are 0.1-0.5 mm diam., subepidermal to erumpent, abaxial, pulvinate and yellowish-brown. Teliospores are 22-50 x 14-28 μm, cylindrical to ellipsoidal, with a rounded apex, yellowish brown, 2-celled, constricted at the septum and pediculate. Basidia are cylindrical, up to 110 μm long, 6-8 μm wide, hyaline, 4-celled, produced from each cell of the teliospores, apically in upper cell and laterally in lower cell. Basidiospores are globose to pyriform, 8-11 μm, hyaline and smooth.
The primary symptom of myrtle rust is the appearance of yellow pustules (uredinia) on the upper and lower leaf surfaces of Myrtaceae, with more tending to be found on the lower surface. Pustules can also be found on stems, fruit and flowers. Slightly darker mustard-coloured pustules may indicate the teliospore stage of the fungus. After 1-2 weeks, the pustules begin to turn pale grey. From this stage, it is difficult to distinguish rust lesions from insect damage or other necrosis.
Related invasive species
- Austropuccinia psidii
Related Farm Practice
- Defoliation
- Production
- Hosts
Austropuccinia psidii is a rust fungus with a wide and expanding host range within the Myrtaceae, with over 440 host species currently known (Carnegie and Lidbetter, 2012;Morin et al., 2012;Pegg et al., 2014). Like many rusts, urediniospores of A. psidii can be wind-dispersed over long distances. Viable spores have been detected on clothing and personal effects following visits to rust-affected plantations (Langrell et al., 2003), 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. 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). 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 (Coutinho et al., 1998) and New Zealand (Kriticos and Leriche, 2008).
A. psidii attacks young, soft, actively-growing leaves, shoot tips and young stems. Fruit and flower parts are also susceptible. The first signs of rust infection are tiny spots or pustules. These symptoms can appear 2-4 d after infection. Symptoms can vary depending on the host species, susceptibility level within a host species, and age of the host leaf. After a few days, the pustules or uredinia erupt with the production of distinctive, yellow urediniospores. The infected area spreads radially outwards and multiple pustules eventually merge and coalesce with age. Secondary infections can occur within days but are usually confined to new young tissue, shoots and expending foliage. Left untreated, the disease can cause deformed leaves, heavy defoliation of branches, dieback, stunted growth and even plant death.
Carnegie and Lidbetter (2012) provide the most recent published host list for A. psidii, based on extensive searches of overseas records (see references therein) as well as the then current host records from surveys in Australia obtained from State Government agencies. The taxonomy of Myrtaceae is in a constant flux, with accepted naming of genera and species often controversial (even within a country). Carnegie and Lidbetter (2012) use the classification according to Govaerts et al. (2011), and as such altered original published host names to fit this classification where necessary (providing synonyms for many). The Australian records have since increased based on host testing (Morin et al., 2012;Sandhu and Park, 2013;F. Giblin, University of the Sunshine Coast, Queenland, Australia, unpublished data, 2014) and increased detections during field surveys (Pegg et al., 2014), with new hosts also from New Caledonia (DAVAR Nouvelle-Calédonie, 2014). This brings the current global host list for A. psidii to 445 species, in 73 genera and 16 tribes of Myrtaceae. A proportion of these hosts are known only from host testing. For example, in Australia there are 346 host species (56 genera) known (Carnegie and Lidbetter, 2012;Morin et al., 2012;Pegg et al., 2014), with approximately 116 of these known only from host testing (Morin et al., 2012;Sandhu and Park, 2013;F. Giblin, unpublished data, 2014).
For reasons of space, the Host plants and Other Plants Affected table in this datasheet lists only the genera affected and the species for which full datasheets are included in Compendia.
The most highly susceptible species recorded to date are Syzygium jambos, Eugenia reinwardtiana, Agonis flexuosa, Gossia inophloia, Melaleuca quinquenervia, Rhodamnia rubescens, R. maideniana, R. angustifolia, Chamelaucium uncinatum and Decaspermum humile (Pegg et al., 2014) and Rhodomyrtus psidioides (Carnegie and Cooper, 2011).