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The incidence, severity and possible causes of canker disease in Corymbia calophylla (marri) trees in south west of Western Australia

Paap, Trudy (2001) The incidence, severity and possible causes of canker disease in Corymbia calophylla (marri) trees in south west of Western Australia. Honours thesis, Murdoch University.


To determine the extent, severity and cause of cankers in Corymbia calophylla (marri) throughout the south west of Western Australia, field work was undertaken at three regions: Brunswick Junction, Perup and north west of Walpole. At each region, transects of stands of remnant vegetation (on private property or road verges), and in state forest allowed quantification of canker incidence, site and tree features. Cankers were sampled for potential fungal pathogens at these six sites, as well as cankered trees in state forest and roadsides east of Rocky Gully, and a railway siding in Balingup.

Cankers were severe and widespread throughout the south west of W. A., and of the 300 trees assessed, 31.7% (95 trees) had at least one canker. Cankers were present on trunks, main branches, smaller branches, and as small lesions on twigs. Girdling cankers had caused tree death at five of the six sites. There were significantly (p<0.05) more cankered trees at the remnant sites than in state forest (47.3% and 16%, respectively).

Though tree height, diameter at breast height (DBH) and crown ratings and health could be used as significant predictors of cankers occurring on a particular tree, these factors accounted for only 14% of the variation in the number of cankers predictable from the linear combination of the tree features (R2=0.141, F=16.78 (3, 306), p<0.0001).

There was no significant (p>0.05) relationship between rainfall or trunk aspect and canker frequency.

Damage caused by gall forming weevils, leaf feeding insects and shoot death contributed to tree decline, however, there was no correlation (p>0.05) between the presence of insect galls and cankers.

Twenty two species of fungi were isolated from the 42 intensively sampled trees. Four fungal species, Cytospora eucalypticola (55%), Endothiella sp. (50%), Favostroma sp. (37.5%) and a Fusarium-like species (12.5%) were regularly isolated from cankers in all regions. A Ramularia sp. was isolated from leaves of Corymbia maculata at the Brunswick Junction property site, and from marri shoots and leaves in the adjacent state forest. A selective medium (half strength potato dextrose agar with the addition of 10 g/L chloramphenicol and 10 g/L rose bengal) was developed for the isolation of Ramularia, and while it provided an efficient method for isolation of the pathogen from infected leaf material, its effectiveness with infected woody tissue remains to be verified.

Examination of the Endothiella sp. isolates showed conidia size was similar to that of Endothiella gyrosa, however this identification must be verified with molecular work. A great deal of variation was observed in colony morphology. Attempts to catogorise isolates on the basis of colony morphology were unsuccessful, as it was highly unstable, even within colonies from single spore isolates. A vegetative compatibility grouping (VCG) trial showed extremely high genetic variation, with 23 VCGs from 24 isolates, and isolates from more than one VCG present from a single canker. The genetic variation is only tentatively described as large, and molecular tools are recommended for further population studies.

An inoculation trial using three Endothiella sp. isolates from marri cankers into marri seedlings defoliated at 0, 50 and 100% showed the fungus caused significantly (p<0.05) larger lesions than controls in all but one isolate/defoliation regime combination. The pathogen induced the largest lesions in the nonde- _,. foliated control seedlings, contrary to what had been anticipated. Lesions of nondefoliated seedlings were significantly (p<0.05) larger than those of 100% defoliated seedlings, and visibly, though not significantly (p>0.05) larger than those of 50% defoliated seedlings. Variation in pathogenicity was observed between the Endothiella sp. isolates.

A winter field inoculation trial using two to three year old marri saplings showed that after four months, isolates of Cytospora eucalypticola, Botryosphaeria sp. and especially Endothiella sp. were pathogenic. There were significant (p<0.05) differences in pathogenicity between the two Endothiella sp. isolates, with one being particularly aggressive, causing long and sometimes girdling lesions. Of two Favostroma isolates, only one produced significantly (p<0.05) longer, but not wider, lesions than controls. A Fusarium-like species, and an isolate of Ramularia from the Department of Conservation and Land Management (CALM) culture collection did not produce significant lesions.

While it is tentatively suggested that there is a current canker epidemic being caused by Endothiella sp., the possibility of the involvement of a fastidious primary pathogen is not ruled out. It also appears likely that the decline of marri is a disease of complex etiology, with a range of contributing factors including insect damage, possibly introduced and endemic pathogens, and a range of environmental stresses. There is a pressing need for further research to determine the causes, and to enable the development of strategies for the control and management of marri decline.

Item Type: Thesis (Honours)
Murdoch Affiliation(s): School of Biological Sciences and Biotechnology
Notes: A digital copy of this thesis is not available. Your library can request a copy from Murdoch University Library via Document Delivery. A fee applies to this service.
Supervisor(s): McComb, Jen, Hardy, Giles and Shearer, Bryan
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