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Analysis of variability among isolates of Phytophthora cinnamomi Rands from Eucalyptus marginata Donn ex Sm. and E. calophylla R. Br. based on cultural characteristics, sporangia and gametangia morphology, and pathogenicity.

Hüberli, Daniel (1995) Analysis of variability among isolates of Phytophthora cinnamomi Rands from Eucalyptus marginata Donn ex Sm. and E. calophylla R. Br. based on cultural characteristics, sporangia and gametangia morphology, and pathogenicity. Honours thesis, Murdoch University.

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On the basis of isozyme, morphological and pathological studies, genetic variability among isolates of Phytophthora cinnamomi in Australia has been reported to be low. Recently there has been a report which indicates that isolates vary in pathogenicity in clonal Eucalyptus marginata. However, no studies have examined variability in a number of life history characteristics using the same P. cinnamomi isolates. In the present study, 72 isolates of P. cinnamomi were examined for variability, based on the following characteristics: radial growth rates on different agar media and at different temperatures; sporangial and gametangial morphology; and pathogenicity in nonclonal E. calophylla and E. marginata and clonal E. marginata. Sixty-six isolates had been isolated from E. calophylla and E. marginata from the Huntly, Jarrahdale and Willowdale minesites (Alcoa of Aust. Ltd.). The remaining 6 isolates had been isolated from a range of plant species and were obtained from Old’s and Dudzinski’s (CSIRO, Canberra) culture collection (ODCC).

Radial growth rate studies on cleared V8-juice agar (V8A) and potato dextrose agar (PDA) at 15°C and 24°C showed that the P. cinnamomi isolates varied significantly (p=0.05). Isolates from Willowdale had slightly higher growth rates than the other isolates. Isolates from ODCC generally had the lowest radial growth rates. Willowdale isolates were less thermosensitive than the Jarrahdale isolates. The majority of the 72 isolates had radial colony morphologies, while only a few had semi-rosette and rosette morphologies.

The pathogenicity of 72 isolates in excised branches of non-clonal E. marginata and E. calophylla was examined. This was based on lesion growth rates, growth extension beyond the lesion margin (EBL) and the time until the lesion was first observed (lag phase). Considerable variation in pathogenicity was observed among the 72 isolates The most pathogenic group of isolates were those from Willowdale.

Ten isolates, 6 highly pathogenic and 4 of low pathogenicity, were selected for further studies in excised branches, intact branches in the field and in roots of susceptible (SS) and resistant (RR) E. marginata clones. In excised branches, lesion growth rates were lower in the RR clone than in the SS clones. However, 4 isolates produced lesion extensions (mm) in the RR clone that were almost identical in at least one SS clone. This suggests that these 4 isolates were able to overcome the resistance mechanisms of the RR clone. In the field and root experiments, there was no distinction of growth rates between the SS and RR clones.

Examination of gametangia of the 10 isolates selected for variation in pathogenicity revealed that 7 were capable of producing oogonia with paragynous and amphigynous antheridia. This is the first recording of paragynous antheridia in P. cinnamomi. The gametangial, sporangial and chlamydospore dimensions were significantly (p=0.05) different among the 10 isolates. There was also wide variability in zoospore production among the isolates. Cluster analysis of the 72 isolates based on morphological and pathological phenotypes distinguished 4 groups of isolates that corresponded to geographical location for 3 groups and the fourth contained 4 out of the 6 isolates from ODCC. There was no distinction of isolates based on the host species they were originally isolated from.

In conclusion, this study has shown that variability in morphological, physiological and pathological characteristics occurs among isolates of P. cinnamomi. This raises the question that these isolates may differ genetically.

Item Type: Thesis (Honours)
Murdoch Affiliation(s): School of Biological and Environmental Sciences
Supervisor(s): Hardy, Giles and Tommerup, Inez
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