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The role of mycorrhizal fungi in Eucalyptus gomphocephala (Tuart) health

Ishaq, Lily (2014) The role of mycorrhizal fungi in Eucalyptus gomphocephala (Tuart) health. PhD thesis, Murdoch University.

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Eucalyptus gomphocephala DC (tuart) is a woodland tree endemic to the Swan Coastal Plain of Western Australia. The species is of importance for ecological, economic and cultural values. However, E. gomphocephala is in severe decline and the cause of the decline is poorly understood. Due to their important role in ecosystem function, mycorrhizal fungi are hypothesized to play a role in the health status of E. gomphocephala. Recently a fungal pathogen, Phytophthora multivora has been implicated in tree decline.

In vitro and glasshouse trials were conducted to examine the impact of P. multivora on ectomycorrhizal (ECM) fungi and E. gomphocephala seedlings. Phytophthora multivora was negatively affected by Pisolithus albus (Cooke and Massee) Priest and a Pisolithus sp., and the ECM fungi were not/or only slightly impacted by P. multivora. The ECM fungal inoculation did not increase shoot growth of E. gomphocephala seedlings in the presence of P. multivora, however, root growth increased and shoots appeared healthy throughout the trial.

Intact soil cores for bioassay were collected from 12 sites with E. gomphocephala canopy condition ranging from healthy to declining. Sites were classified into three classes; healthy, moderately healthy and declining based on total crown health index. In each site, four trees were randomly selected and eight intact soil cores (0 - 20 cm depth) were collected. Soil samples for arbuscular mycorrhizal (AM) fungi spore assessment and chemical analysis were collected adjacent to the intact soil core. Sites differed for seventeen soil chemistry parameters, and nine variables measured were significantly related to the canopy condition of E. gomphocephala in the field. Although spore density of AM fungi differed between the sites there was no relationship between spores and canopy condition of E. gomphocephala.

A bioassay pot experiment with E. gomphocephala as the trap plant was set up using intact soil cores to evaluate inoculum potential of mycorrhizal fungi. Soil cores for AM fungi spore assessment were taken one month prior to harvest and plants were harvested at seven months. Roots were assessed for mycorrhizal colonization and the relationship between mycorrhizal colonization and crown health was evaluated. ECM fungi colonization dominated seedling roots produced in soils taken from under healthy canopies whereas AM fungi dominated seedling roots in soils taken from under trees with declining canopies. From the seventeen soil chemical properties measured, eight and ten variables were related to ECM and AM colonization, respectively.

Molecular analysis was undertaken to evaluate mycorrhizal communities associated with seedling roots from the bioassay trial. This confirmed observations on root colonization, namely there were more ECM fungi associated with seedling roots grown in soil collected from under healthy canopies, and more AM fungi were found in seedling roots grown in soil collected from under declining canopies. Also, there were more saprophytic fungi for seedling roots grown in soil collected from healthy canopies than for seedlings in declining canopy soil, but the reverse was the case for pathogenic and root rot fungi.

A second glasshouse trial was undertaken to assess mycorrhizal colonization over time using soils taken from contrasting sites. A similar trend in dominance of ECM in seedlings grown in soil collected from under a healthy canopy was observed even when the seedlings were still quite young. For example, at 5 weeks root tips of seedlings in a healthy site soil were 24% colonized by ECM fungi compared with 3% in the declining site soil.

This is the first study to investigate mycorrhizal formation in relation to the health status of E. gomphocephala. The low ECM fungal colonization of E. gomphocephala seedlings grown in declining sites soil might indicate reduced ECM inocula in the sites where the soil were collected. Reasons for the changes in populations of mycorrhizal fungi under declining trees are discussed. This research now needs to be extended to the field by investigating colonization patterns in seedlings and also the molecular analysis of mycorrhizal fungi populations in sites with a range of canopy conditions. An approach for managing health of E. gomphocephala, thus could be gained in the future.

Publication Type: Thesis (PhD)
Murdoch Affiliation: School of Veterinary and Life Sciences
Supervisor: Dell, Bernard, Hardy, Giles and Barber, Paul
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