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Detection, assessment, and management of Phytophthora species in an urban forest

Khdiar, Mohammed (2018) Detection, assessment, and management of Phytophthora species in an urban forest. PhD thesis, Murdoch University.

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Urban forests contribute to human well-being and environmental health and function, and this role will increase as urbanization continues to grow. Due to anthropogenic activities, urban forests are considered a suitable starting point for invasive pathogens. Members of the genus Phytophthora are important pathogens involved in tree and forest declines worldwide. To date, there are approximately 160 formally described Phytophthora species, with 50% of these species described in the last decade. There are many factors contributing to these recent descriptions, including introductions from the nursery trade, increased studies in forests and natural environments, and an increase in the number of Phytophthora scientists globally. Improved molecular tools to detect and identify Phytophthora species (as like other genera of the oomycetes) has also had a profound effect on the increased number of species that have recently been described. Within urban forests, trees are under considerable stress from polluting agents, mechanical damage, and other human activities and are often in sub-optimum health and are thus more vulnerable to pathogens such as Phytophthora.

Two hundred and thirty-six soil and root sites were sampled from declining native Australian trees in 91 parks and nature reserves in the City of Joondalup, Perth Western Australia. Metabarcoding DNA detected forty-five Phytophthora species. Phytophthora multivora was isolated the most frequently while P. cinnamomi was the fifth most frequent. Seven species P. capsici, P. sp. pecan, P. fluvialis, P. gonapodyides, P. sp. walnut, P. erythroseptica and P. fallax were isolated only once. In contrast, only P. nicotianae, P. multivora, P. boodjera and P. arenaria were isolated into the culture based on rhizosphere baiting. A range of environmental factors was examined to determine if Phytophthora communities were related to specific environmental factors. The effect of environmental factors ranged from no effect (land class, plant diversity, park type, soil type, soil unit and canopy health) to a slight effect (canopy cover and park size) on the overall Phytophthora community. When looking at the incidence of P. cinnamomi, park type was significant, while for P. multivora, soil type, soil unit, and park size were significant drivers.

A pathogenicity test was conducted on excised branches from 15 tree species underbark inoculated with 19 Phytophthora species. All Phytophthora species formed lesions in Eucalyptus marginata and Corymbia calophylla, while Fraxinus excelsior was resistant to 7 of the 19 Phytophthora species. Six Phytophthora species were pathogenic to all 15-tree species, whereas Phytophthora versiformis formed lesions in only three tree species. Phytophthora cinnamomi, P. pseudocryptogea, and P. citrophthora were the most pathogenic species causing the largest lesions in most of these tree species. These results show that Phytophthora species potentially play an essential role in the declining health of urban trees and indicate the importance of managing Phytophthora in urban forests.

Two chelate complexes [Zn (Val)2(bipy)] and [Ca(Val)2(bipy)] were developed and screened for their potential as fungicides to control Phytophthora species. These complexes were examined using three spectroscopic methods (X-ray crystallography, ultraviolet rays–visible spectra and Fourier-transform infrared spectroscopy). The complexes were pseudo-octahedral in structure, and both complexes had the same patterns of crystal structure symmetry. The yield of the calcium chelate and zinc chelate of basic materials were 81% and 50%, respectively. The activity of these complexes with or without the addition of phosphite against P. cinnamomi in vitro and in planta was examined. In vitro, the effect of the chemical treatments was assessed by measuring the EC50 of mycelial biomass in a liquid medium with 0-160 μg/ml (0-0.16g/l) of each complex. Phytophthora cinnamomi was highly sensitive to the Ca chelate alone and Ca chelate + phosphite, but not to the zinc chelate. Two plants species Banksia grandis and Eucalyptus marginata were sprayed with three concentrations (0, 0.25 and 0.5%) of each chemical treatment (phosphite, Ca chelate, Zn chelate and Ca chelate + phosphite) and were inoculated with P. cinnamomi together with a non-inoculated control. Ca chelate significantly reduced lesion development of P. cinnamomi compared to phosphite alone. Whilst Ca chelate (0,25%) + phosphite (0,25%) gave the highest improvement in reducing the effects of P. cinnamomi in both plant species. None of the Zn chelate applications were effective. The Ca chelate has the potential to be used as a fungicide to control Phytophthora species. These results suggest that Ca chelate may be a suitable alternative to phosphite to control Phytophthora species.

Item Type: Thesis (PhD)
Murdoch Affiliation(s): School of Veterinary and Life Sciences
United Nations SDGs: Goal 13: Climate Action
Goal 15: Life on Land
Supervisor(s): Hardy, Giles
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