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Lifecycle, biology and diversity of Puccinia boroniae in Western Australia

Driessen, Susanna (2005) Lifecycle, biology and diversity of Puccinia boroniae in Western Australia. PhD thesis, Murdoch University.

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The rust fungi (Uredinales, Basidiomycota) are an expansive and diverse group of fungal species, consisting of approximately 7000 different species in over 160 different genera. Fungi of the genus Puccinia represent a large proportion of these rust fungi, many species of which are well known for their role in causing massive yield and subsequent economic losses in agricultural crops worldwide. Puccinia boroniae is one such rust fungus and is a significant pathogen of several species of Boronia (Rutaceae), a native Australian wildflower grown commercially in Western Australia as a cutflower. Complete control of the rust pathogen is rarely achieved using chemical fungicides. Improving the level of disease control is vital for the long-term sustainability and future growth of the Boronia industry, and requires an understanding of the pathogen. The objectives of this thesis were to investigate aspects of the epidemiology, the biology and the diversity of P. boroniae in Western Australia, providing a broad understanding of the pathogen, which in turn could be employed to improve disease control.

The lifecycle of P. boroniae was conclusively shown to be microcyclic by artificial inoculation of Boronia heterophylla with basidiospores released from germinating teliospores suspended over the host plant. Telia developed on the leaves within 21 days, with no intermediate rust spore stages (pycnial, uredial or aecial) observed. Rarely, low numbers of pycnia of P. boroniae were observed on field specimens collected from leaves of B. megastigma cultivated at one commercial floriculture plantation. This was the first record of pycnia of P. boroniae; however, as pycnia were not observed on other host species or plantations, or formed during controlled inoculation trials, their functional role in the lifecycle is currently unresolved.

Telia were subepidermal, erumpent and pulvinate, amphigenous on leaves, stems and parts of developing flower buds, and generally persistent year round. Intracellular hyphae resembling monokaryotic haustoria (M-haustoria) were observed in leaf mesophyll cells beneath and adjacent of telia. Occasionally Sphaerellopsis filum (teleomorph Eudarluca caricis), a known mycoparasite of rust fungi, was observed on the telia. Under favourable conditions, teliospores germinated immediately without a period of dormancy, with fully mature basidiospores formed within 3-4 h after telia were exposed to moisture. Basidial development in P. boroniae was unusual, in that only one basidiospore was formed from each germinating teliospore cell. Immature teliospores were initially binucleate undergoing karyogamy to form a single large (presumably diploid) nucleus that migrated into the developing metabasidium. Both binucleate and tetranucleate metabasidia were observed, with mature uninucleate, binucleate and tetranucleate basidiospores present. At this stage, more research is required to understand the complete nuclear behaviour during teliospore germination. The morphology of the pycnial stage was similar to other Puccinia species, being ampulliform, subepidermal, amphigenous and arranged in small clusters on leaves of B. megastigma. However, the spine-like periphyses protruded through stomata as apposed to penetrating the leaf epidermis.

Environmental conditions favouring the formation and dispersal of basidiospores were assessed in vitro and under field conditions with a spore catcher. Under field conditions, basidiospores were captured from February-August 2004, with peak numbers and daily incidence occurring during autumn (April/May) when the average temperature range was 9.1-22.6 degrees C. Daily basidiospore numbers were positively correlated with minimum daily temperature and total daily rainfall. A distinct diurnal periodicity of release was observed, with numbers peaking on average between 02:00 and 05:00 hrs. The hourly release of basidiospores was positively correlated with relative humidity and negatively correlated with temperature and evaporation. This data was in agreement with the in vitro experimentation, which showed that basidiospore formation occurred between 10-25 plus-minus 1 degrees C (apparent optimal temperature of 15-20 plus-minus 1 degrees C) with telia incubated in continuous darkness promoting a greater number of basidiospores.

The level of genetic variation of P. boroniae in Western Australia was assessed by PCR-RFLP of the nuclear ribosomal intergenic spacer 2 (IGS2) region. Two RFLP profiles were observed, separating three specimens (Group 1) from the remaining population (Group 2). Sequence analysis indicated that point mutations at endonuclease recognition sites were responsible for the changes in RFLP profile. Group 2 specimens had been collected from the same host species (B. megastigma) and plantation, and it is suggested that the variant specimens may constitute a subspecies of P. boroniae, isolated by geographic location and possibly host (cultivar) specificity. Further analysis, primarily pathogenicity trials, is needed to confirm this.

This study has improved our knowledge regarding the rust fungus P. boroniae and has laid strong foundations for future research into several aspects of the biology, epidemiology and population variation. The implications of the key findings of this research, with an emphasis on the management of P. boroniae in commercial situations, are discussed.

Item Type: Thesis (PhD)
Murdoch Affiliation(s): School of Biological Sciences and Biotechnology
Supervisor(s): Hardy, Giles
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