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Alternaria blight of Paulownia in Western Australia: biology, taxonomy, pathogenicity, diversity and Paulownia varietal resistance

Ray, Jane D. (2004) Alternaria blight of Paulownia in Western Australia: biology, taxonomy, pathogenicity, diversity and Paulownia varietal resistance. Honours thesis, Murdoch University.

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Paulownia plantations are a relatively new tree crop in Western Australia (WA), consequently very little is known of the diseases that affect the trees here. Tree decline associated with fungal plant pathogens has started to be a concern.

To determine the extent, severity and cause of tree decline in plantation grown Paulownia in Western Australia, field work was undertaken at eight commercial plantations: Kiri Park, Gingin, Coorow, Moora, Blue Lakes, Beenyup, Nowergup, and Manjimup. At each plantation, a targeted survey was conducted for the symptoms of 'black leaf', 'brown leaf', trunk canker, and petiole lesions, the most common disease symptoms in Paulownia plantations. Symptomatic plant tissues were collected and potential fungal pathogens were isolated and identified. This was followed by pathogenicity tests on trunks and leaves of Paulownia to confirm Koch's
Postulates. Morphological and molecular techniques were utilised to identify and characterise the cause of 'black leaf', 'brown leaf', trunk canker, and petiole lesions as Alternaria alternata. In addition, trials were undertaken to investigate the resistance of fifteen Paulownia genotypes to Alternaria disease.

This study clearly showed that A. alternata and its symptoms are widespread in commercially grown Paulownia in WA. During the survey almost 100% of Paulownia trees showed symptoms of 'black leaf' and 'brown leaf' regardless of tree health or tree age at Kiri Park, Gingin, Coorow, Moora, Blue Lakes, Beenyup, and Nowergup. Trunk cankers on Paulownia were observed at the Kiri Park, Coorow, and Gingin plantations. Paulownia tissue with symptoms of 'black leaf, 'brown leaf', petiole lesions, and trunk cankers were infected with A. alternata (100%), while (62%) of asymptomatic trunk samples and (70%) of asymptomatic leaf samples were also infected with A. alternata.

Twenty-seven of the thirty-four Alternaria isolates (79%) tested in vitro caused lesions on wounded leaf tissue, the remainder were non-pathogenic.

Wounded trunk inoculation of Paulownia with A. alternata isolates resulted in lesions. A. alternata was also able to directly penetrate unwounded trunks of juvenile Paulownia trees under experimental conditions, and colonise the tissue as an endophyte or latent pathogen. Canker development in the field was most likely attributed to a synergistic effect of stress or wounding and A. alternata. Thus, A. alternata isolates from diseased Paulownia can be categorised as a stress or latent pathogen.

There were significant differences in the susceptibility of the 15 Paulownia genotypes to Alternaria 'brown leaf' disease. Genotypes KP16, KP32, and KP8 were less susceptible to 'brown leaf' damage, and to A. alternata in vitro, whilst, the genotypes KP15, KP21, KP12, and KP11 were more susceptible to these factors. The in vitro screening of Paulownia genotypes suggested differences in the susceptibility of the genotypes to Alternaria disease, however, these results did not correlate with the field data for 'black leaf' or 'brown leaf' disease. The findings are important in that it indicates in vitro screening is not a satisfactory method for selection of resistant cultivars to A. alternata.

The taxonomy of small-spored Alternaria spp. has been a contentious issue. However, through multiple gene genealogies, and RAMs analysis conducted in the current study it is now known that the species of A. tenuissima, A. arborescens, and A. alternata are the same fungal species, and should all be referred to as A. alternata, with host-specific toxin producers termed formae speciales or pathotypes. This is the first study to show this, and has important ramifications on Alternaria taxonomy.

The ITS rDNA sequence analysis suggests that all small-spored Alternaria species form a monophyletic clade with 100% bootstrap support. It appears that the small-spored chain forming species are more closely related to the genus Ulocladium than to other larger spored Alternaria spp. or the morphologically similar A. infectoria.

This is the first study of morphological characterisation of A. alternata isolates from Paulownia in WA. These A. alternata isolates exhibit large amounts of variation in colony colour and sporulation-pattern morphology, and the findings do not agree with previously described morphological groupings. The results of the current study show that for WA isolates of A. alternata, and the morphologically similar but distinct species of A. infectoria cannot be separated using the sporulation-pattern technique and colony colour. Accurate identification of A. infectoria, and A. alternata is easily confirmed by sequencing of the ITS rDNA.

This study has shown that A. alternata plays a major economic role in Paulownia decline in Western Australia. As this is the first study of its type on diseases of Paulownia in WA it has highlighted areas for further research into Alternaria diseases of Paulownia, Paulownia genotype resistance to A. alternata, and taxonomy of Alternaria spp.

Item Type: Thesis (Honours)
Murdoch Affiliation(s): School of Biological Sciences and Biotechnology
Notes: A digital copy of this thesis is not available. Your library can request a copy from Murdoch University Library via Document Delivery. A fee applies to this service.
Supervisor(s): Hardy, Giles, Burgess, Treena and Malajczuk, Nicholas
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