Botanic Gardens Trust, Sydney, Australia

Plant Pathology and Mycology 2011-2012

Matthew Laurence - Diagnostics and Facilities Coordinator; PhD Candidate and Dr Edward Liew - Manager Plant Pathology

The Plant Pathology and Mycology team continued research activities in documenting and describing foliar diseases of members of the Proteaceae and Myrtaceae, describing new species of the genus Fusarium, understanding the biogeography and evolution of soil inhabiting and endophytic species of Fusarium, studying movement and distribution of economically significant Fusarium pathogens and investigating disease management strategies for a range of diseases incited by the water mould Phytophthora. Work progressed further on modelling disease risks of Phytophthora Dieback in the Greater Blue Mountains World Heritage Area (GBMWHA). Two students completed their PhD projects during the year; Ameera Yousiph and Lucas Shuttleworth submitted their thesis on Snow Pea Wilt and Chestnut Rot, respectively. The team also embarked on a collaborative project to establish the phylogenetic relationships of Puccinia psidii (eucalyptus rust, myrtle rust or guava rust) within the Pucciniales.

Research highlight

The Fusarium oxysporum Species Complex (FOSC) is one of the most intensively studied group of fungi because of their devastating economic impact in causing wilt diseases in cultivated crops. It is increasingly acknowledged as a significant human pathogen in causing fusariosis. Yet this fungus is primarily non-pathogenic and is widely distributed in non-cultivated ecosystems ranging from the permafrost soils of the Arctic to the sands of the Sahara. In natural ecosystems F. oxysporum was thought to be primarily non-pathogenic. However, genetic characterisation of strains isolated from recent outbreaks of cotton wilt in Queensland found that they were distinct from international strains and have evolved together with natural populations of F. oxysporum in Australia. Additional phylogenetic research into the origins of pathogenic strains led to the realisation that F. oxysporum is made up of multiple undefined species.

Defining species in Fusarium has been historically problematic due to the lack of morphological characters and difficulties in defining biological species based on experimental laboratory crossings. DNA sequencing technology has enabled an alternative approach for establishing biological species boundaries through the phylogenetic analysis of house keeping genes. The idea is that recombination within a lineage will create conflicts between gene (phylogenetic) trees, with the transition from conflicts to congruence indicating species limits. The identification of the tree node that most likely represents this transition defines the species boundary (Figure 1).

We applied this method to F. oxysporum using six house keeping loci and resolved two species within the complex. The first phylogenetic species is hypothesised to be ancestral and may have originated in Australasia due to the high levels of novel genetic diversity when compared to international strains. Interestingly the indigenous cotton wilt pathogen discovered is a member of this newly defined species and comparative sequence analysis of a pathogenicity element indicates a long divergence time from pathogens in the other species. The second phylogenetic species contains the majority of plant and human pathogenic strains and, based on levels of genetic diversity, is likely to have evolved outside of Australasia, perhaps anthropogenically introduced through agriculture.

This work formed part of Matthew Laurence’s PhD project. Matt has now submitted his thesis! Further investigations on the population structures of these two species are underway. 

 

 

 

 

Figure-1---Ed-Liew-Pathology
Figure 1. Diagram showing the phylogenetic approach of defining biological species boundaries by identifying species limits represented by conflicts and congruence among gene trees. Ref: Taylor et al. 2000

Figure-2---Ed-Liew-Pathology
A site of minimal human disturbance within Carnarvon Gorge National Park, Queensland, from which Fusarium oxysporum was isolated.
Photo: Sarah Dunstan

Figure-3---Ed-Liew-Pathology
Matt Laurence collecting soil samples in Shannon National Park, Western Australia. Photo: June Laurence

Figure-4---Ed-Liew-Pathology
A moment of repose and refreshment for Matt Laurence in Karijini National Park, Western Australia after a hard day’s work collecting soil samples for Fusarium oxysporum isolation. Photo: Emma Laurence