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Role of feral pig (Sus scrofa) in the dissemination of Phytophthora cinnamomi in South-western Australia

Li, Andrew (2012) Role of feral pig (Sus scrofa) in the dissemination of Phytophthora cinnamomi in South-western Australia. PhD thesis, Murdoch University.

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Phytophthora cinnamomi is a soil-borne plant pathogen that causes dieback, a disease that devastates many native vegetation ecosystems in Australia, particularly in south-west Western Australia. Feral pigs have long been implicated as vectors in the spread of this introduced plant pathogen due to their contact with infested soil and foraging habits. This study aimed to investigate the potential for feral pigs to disseminate P. cinnamomi and to determine their role in the spread of dieback.

Feral pigs trapped in three sampling areas within the northern jarrah (Eucalyptus marginata) forest of south-west Western Australia were sampled for the presence of P. cinnamomi. Faecal (n=208) and soil samples (n= 140) were collected from trapped pigs. In addition, 374 faecal and 36 soil samples were also collected from sites frequented by feral pigs. Phytophthora cinnamomi was not recovered from any of the faecal or soil samples. However saprophytic pathogens such as Mucor and Fusarium spp. were detected in the faeces and Pythium spp. was also detected in the soil samples, suggesting that feral pigs can act as vectors for the spread of soil-borne pathogens.

Stomach contents from 100 feral pigs trapped across the three sampling areas were analysed to investigate the proportion of P. cinnamomi susceptible plant matter present in the feral pig diet. A high frequency of plant material (85%) was found in the pig stomachs, of which 25.8% consisted of subterranean plant structures such as roots and tubers. Underground fruiting bodies of ectomycorrhizal fungi belonging to the genus Rhizopogon were also a significant food item. There was no statistically significant preference detected for food items by pigs between the three sampling areas, regardless of sex and/or month of capture. However, older and larger pigs consumed significantly more bark (p= 0.0002).

To further investigate the potential for P. cinnamomi to survive passage through the pig digestive tract a feeding trial was undertaken. Phytophthora cinnamomi inoculated millet (Panicum miliaceum) seeds, pine (Pinus radiata) plugs, and Banksia leptophylla roots were fed to pigs and subsequently recovered after passage. The viability of P. cinnamomi inoculated plant materials post digestion ranged from 25.5% to 98.3%. Detection for P. cinnamomi presence in the materials via qPCR confirmed a decrease in P. cinnamomi DNA with increasing time to passage. These investigations demonstrated that plant material infected with P. cinnamomi can remain viable following passage through the pig digestive tract suggesting that the plant material may provide protection for P. cinnamomi against the adverse conditions of the pig digestive tract. Subsequently, plant infection trials using infected pine plugs passaged through the pig digestive tract highlighted that material passaged 7 days after initial consumption was capable of infecting healthy susceptible plants. This provides evidence that feral pigs have the ability to act as a vector for P. cinnamomi through the ingestion of infected plant materials.

A species-specific fluorescent in situ hybridization (FISH) assay was developed to enable the examination of P. cinnamomi within plant tissues. The probe was found to be specific for P. cinnamomi when tested against other Phytophthora, Pythium and enteric bacteria species. Using the FISH assay, the location of P. cinnamomi structures were detected within a variety of plant materials such as millet seeds, pine sections and root samples. Phytophthora cinnamomi structures such as hyphae and chlamydospores were found in the epidermal layer of millet seeds and within the axial rays of pine that were recovered after passage from the feeding trial. This aided understanding of how viable P. cinnamomi were able to survive passage within these plant materials. In addition, the FISH assay was also successfully applied to both laboratory-cultured and naturally infected plant roots enabling detection of the pathogen in the intracellular and intercellular spaces of roots. The assay has proven to be a useful tool in the detection of P. cinnamomi structures within plant tissues.

In conclusion, this study provides evidence that, whilst the potential consequences of pig-vectored dispersal of P. cinnamomi are high, the likelihood of feral pigs dispersing the pathogen through transport of infested soil is low. Investigations of their diet composition and the passage of viable P. cinnamomi has established the additional threat that feral pigs could spread ingested P. cinnamomi within organic substrates. This study has also highlighted the fact that there is still much to be learned about the interaction between the feral pig and the plant pathogen. Further research is therefore required to ensure that appropriate management decisions for both species can be made.

Item Type: Thesis (PhD)
Murdoch Affiliation(s): School of Veterinary and Biomedical Sciences
Supervisor(s): Adams, Peter, Williams, Nari, Fenwick, Stan and Hardy, Giles
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