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Interactions between bacteria and Phytophthora cinnamomi Rands

Nesbitt, Harold John (1979) Interactions between bacteria and Phytophthora cinnamomi Rands. PhD thesis, Murdoch University.

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Phytophthora cinnamomi Rands was incubated in lateritic soil and soil leachates containing increasing amounts of jarrah (Eucalyptus mavginata Bonn, ex Sm.) leaf material. The interactions between the bacteria contained in these soils and P. cinnamomi were then examined.

Soils ameliorated with jarrah leaf material contained increased nutrient concentrations compared with the impoverished lateritic soils. The size of the microbial population was also increased with organic amendments and this was apparently responsible for the promotion in lysis of P. cinnamomi incubated in the soils. Sporangial formation in the pathogen was also stimulated, but when the two were compared it appeared that sporangia formed as a survival response to hyphal lysis.

Heterolysis of P. cinnamomi was more rapid in soils incubated at field capacity or below and at a temperature approximating 25°C. Microbially induced lysis of the pathogen was decreased at soil temperatures below 25°C and in waterlogged soils. In addition, autolysis was rapid at a soil temperature of 34°C and in dry soils, confirming field observations that root rot caused by P. cinnamomi is more severe in constantly moist areas and that the pathogen does not survive in hot and/or dry soils.

Sporangial formation in P. cinnamomi incubated at 25°C in soil and soil leachates was studied in detail. A peak in the formation of sporangia was observed in leachates of soils collected from the jarrah forest during winter and summer. This may have been a reflection of the microorganism population in the leachates taking advantage of nutrient accumulation in the forest soil during summer and winter when physical conditions in the field restricted microbial activity. Increased numbers of sporangia were formed in soils containing additions of jarrah leaf material compared with lateritic soil. However, fewer zoospores were released into these leachates than those containing no leaf litter additions. Motility of the zoospores was not reduced by bacterial activity but when encysted they were subjected to the same microbial activity that lysed the hyphae.

The bacterial component of the microbial population appeared to be closely related to lysis of the mycelium of P. cinnamomi. This interaction was studied in detail in colonization, translocation and exudation studies. Translocation rates of metabolites in P. cinnamomi exceeded the growth rate of the pathogen and there appeared to be ample exudation of metabolites from growing hyphae. Bacteria multiplied on and near the hyphal cell surface in the presence of these exudates and applied an external stress to the pathogen, causing lysis. Six bacteria in pure culture did not colonize or lyse P. cinnamomi. These results suggest that the lytic nature of soils is related to the population size of specific species of bacteria which may multiply in intimate contact with the pathogen and not to bacterial numbers in toto.

These experimental results suggest that the promotion of a deep litter layer in the Western Australian jarrah forest may help to suppress the spread of P. cinnamomi induced dieback disease. Litter composition may also play an important role in promoting a bacterial population antagonistic to P. cinnamomi.

Item Type: Thesis (PhD)
Murdoch Affiliation(s): School of Environmental and Life Sciences
Notes: Note to the author: If you would like to make your thesis openly available on Murdoch University Library's Research Repository, please contact: Thank you.
Supervisor(s): Malajczuk, Nicholas and Glenn, Andrew
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