Geoffrey S. Pegg

Ceratocystis species, including two new species associated with nitidulid beetles, on eucalypts in Australia

The genus Ceratocystis includes important fungal pathogens of trees, including Eucalyptus spp. Ironically, very little is known regarding the diversity or biology of Ceratocystis species on Eucalyptus species in Australia, where most of these trees are native. The aim of this study was to survey for Ceratocystis spp., and their possible insect associates, on eucalypts in Australia and thus to establish a foundation of knowledge regarding these fungi on the continent. Collections were made in three states of Australia from wounds on trees, as well as from nitidulid beetles associated with these wounds. Ceratocystis spp. were identified based on morphology and multigene sequence comparisons. Of the 54 isolates obtained, two previously unknown species of Ceratocystis were found and these are described here as Ceratocystis corymbiicola sp. nov. and Ceratocystis tyalla sp. nov. Furthermore, the distribution of Ceratocystis pirilliformis is expanded to include Eucalyptus spp. in Tasmania.

Variable resistance to Quambalaria pitereka in spotted gum reveal opportunities for disease screening

Quambalaria shoot blight, caused by the fungus Quambalaria pitereka, is a serious disease affecting the development of spotted gum (Corymbia citriodora subsp. citriodora, C. citriodora subsp. variegata, C. henryi and C. maculata) plantations in subtropical and tropical Australia. Incorporation of screening for resistance to Q. pitereka into current breeding programs is essential for the future development of plantations using spotted gum and Corymbia hybrids. The aim of this study was to determine whether there is variability in resistance among and within different species provenances and families of spotted gum to infection by Q. pitereka. A secondary aim was to consider whether the origin of seed source is a significant indicator of resistance to Q. pitereka. Assessments were conducted in trials consisting of spotted gum provenances, families and clones, all at the same site with high levels of disease pressure and with optimum climatic conditions for disease development. While all species and provenances of spotted gum could be infected by Q. pitereka, results showed that there are high levels of variability in resistance between and within species, provenances and families, indicating the potential to select for disease resistance. Provenance was shown to be an unreliable indicator of resistance to Q. pitereka.

Quambalaria species: increasing threat to eucalypt plantations in Australia

Spotted gum (Corymbia citriodora subsp. variegata and C. maculata) is a valuable source of commercial timber and suitable for a wide range of different soil types in eastern Australia. The main biological constraint to further expansion of spotted gum plantations is Quambalaria shoot blight caused by the fungus Quambalaria pitereka. Surveys conducted to evaluate the impact of Quambalaria shoot blight have shown that the disease is present in all spotted gum plantations and on a range of Corymbia species and hybrids in subtropical and tropical regions surveyed in eastern Australia. More recently, Q. eucalypti has also been identified from a range of Eucalyptus species in these regions. Both pathogens have also been found associated with foliage blight and die-back of amenity trees and Q. pitereka in native stands of Corymbia species, which is the probable initial infection source for plantations. Infection by Q. pitereka commonly results in the repeated destruction of the growing tips and the subsequent formation of a bushy crown or death of trees in severe cases. In comparison, Q. eucalypti causes small, limited lesions and has in some cases been associated with insect feeding. It has not been recorded as causing severe shoot and stem blight. A better understanding of factors influencing disease development and host–pathogen interactions is essential in the development of a disease management strategy for these poorly understood but important pathogens in the rapidly expanding eucalypt (Corymbia and Eucalyptus spp.) plantation industry in subtropical and tropical eastern Australia.

First report of myrtle rust caused by Austropuccinia psidii on Rhodomyrtus tomentosa (Myrtaceae) from Singapore

Austropuccinia psidii (formerly Puccinia psidii) causes rust on species of Myrtaceae. Since 2006, different genotypes of A. psidii have spread to countries in the Pacific and to South Africa (Roux et al. 2016). The pathogen was recently discovered on Eucalyptus pellita and Melaleuca leucadendra in Indonesia (McTaggart et al. 2015). Three suspected specimens of A. psidii were collected in August 2016 from Singapore on leaves of Rhodomyrtus tomentosa (Myrtaceae), a tree native to southeastern Asia. The symptoms were the same as those caused by this pathogen with unmistakable yellow pustules of uredinia on the young leaves and inflorescences. The specimens were lodged in the PREM herbarium, South Africa (PREM 61592, 61593, 61594). Uredinia and telia were present on the abaxial leaf surface of PREM 61592. Urediniospores were ellipsoid to obovoid, 19 to 24 × 16 to 21 µm; wall 1.5 to 2.0 µm thick, finely echinulate. Teliospores were cylindrical or ellipsoidal, 2-celled, apex rounded, pale yellowish brown, (29–) 35 to 45 (–50) × 16 to 23 µm; wall 1 to 2 µm thick, smooth; pedicel up to 15 µm long. This was morphologically similar to the description of A. psidii in Australia (available at: http://collections.daff.qld.gov.au/web/key/rustfungi/Media/Html/pucciniapsidii.html) (Shivas et al. 2014). A DNA sequence-based identification was made using the internal transcribed spacer (ITS) and large subunit (LSU) regions of ribosomal DNA. The ITS sequences were 99% identical (551 out of 555 base pairs) to KM282154, the epitype sequence of A. psidii (Machado et al. 2015). The ITS and LSU sequences had 99 to 100% identity to specimens from Indonesia, with one degenerate base pair difference in the ITS region (McTaggart et al. 2015). The sequences obtained were deposited in GenBank as KY816929 and KY816930. The genotype of the isolates from Singapore was not determined, but may be the same as in Indonesia (McTaggart et al. 2015), which has the pandemic strain, so named due to its spread through the Pacific. This report of A. psidii in Singapore may indicate the rust will spread further through Southeast Asia, into countries such as Malaysia and Thailand.

The Eucalyptus canker pathogen Chrysoporthe cubensis discovered in eastern Australia

Chrysoporthe cubensis is an important pathogen of commercially planted Eucalyptus species (Myrtaceae) in tropical and subtropical parts of the world where these trees are planted as non-natives. Although the majority of Eucalyptus spp. are native to Australia, Chr. cubensis is not common there and has been reported only once from naturally growing Eucalyptus marginata in Western Australia. Chr. cubensis is able to infect hosts in the Myrtaceae and Melastomataceae other than Eucalyptus, but has not been found on hosts other than Eucalyptus in Australia. Recently, fruiting structures resembling those of Chr. cubensis were discovered on Tibouchina heteromalla, planted as a non-native in the Botanical Gardens in Cairns, northern Queensland. These fruiting structures and resulting isolates were characterised as Chr. cubensis in this study. Pathogenicity studies found that Corymbia spp., including commercially important spotted gum, are susceptible along with E. pilularis and E. dunnii. The discovery of Chr. cubensis on a non-native plant in the northern part of Australia is important as it might imply that the pathogen has been introduced into the country and pose a significant threat to native Eucalyptus forests. Alternatively, this fungus might occur naturally in the area on hosts other than Tibouchina, possibly Eucalyptus or related species, and that Australia forms part of its native range.