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The role of phytophthora in predisposing Corymbia calophylla (marri) to a canker disease

Croeser, Louise (2021) The role of phytophthora in predisposing Corymbia calophylla (marri) to a canker disease. PhD thesis, Murdoch University.

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Corymbia calophylla (marri), a keystone tree species in the global biodiversity hot spot of south-western Australia, is suffering decline and mortality due to canker disease caused by the endemic fungus Quambalaria coyrecup. Phytophthora species, fine root oomycete pathogens, are frequently isolated from the rhizosphere of dying C. calophylla, raising the possibility that a Phytophthora infection predisposes C. calophylla to this endemic canker pathogen by compromising its defence mechanisms. Field surveys conducted across the C. calophylla range, found Phytophthora to be present in the rhizosphere of C. calophylla. Five Phytophthora species (P. cinnamomi, P. elongata, P. multivora, P. pseudocryptogea and P. versiformis) were recovered from healthy and cankered C. calophylla. Phytophthora incidence was significantly higher in anthropogenically disturbed areas. Pot infestation trials were conducted where the C. calophylla plants were inoculated with the recovered Phytophthora species. A significant reduction in root volume and even seedling death were observed, demonstrating that Phytophthora can adversely affect C. calophylla health. In a follow-up trial, C. calophylla plants were inoculated with both P. cinnamomi and Q. coyrecup and subjected to a drought stress treatment. Results indicated that neither P. cinnamomi nor the drought stress treatments exacerbated the pathogenic effect of Q. coyrecup on the plants. During these trials, weekly reflectance spectroscopic measurements with a portable high-resolution spectroradiometer, were also taken to investigate its potential to track biochemical changes in the C. calophylla leaves due to these treatments. Reflectance values displayed differences between treatments, as well as a seasonal trend in the leaves. Bandwidths in the visible and shortwave infrared regions of the electromagnetic spectrum were demonstrated to be important regions for characterising C. calophylla response to the Phytophthora, Q. coyrecup, waterlogging and drought stress treatments. More work is required to identify the optimum wavelengths for C. calophylla. Once the optimum bandwidths have been determined, reflectance spectroscopy measurements can be scaled up to canopy level, using unmanned vehicles or fixed-wing aircraft; thus, aiding in the management of this canker disease in C. calophylla.

Item Type: Thesis (PhD)
Murdoch Affiliation(s): Centre for Climate-Impacted Terrestrial Ecosystems
Harry Butler Institute
Supervisor(s): Burgess, Treena, Hardy, Giles, Admiraal, Ryan, Barber, Paul, Paap, Trudy and Andrew, Margaret
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