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Studies on myrtle wilt


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Packham, JM 1994 , 'Studies on myrtle wilt', PhD thesis, University of Tasmania.

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Nothofagus cunninghamii, or myrtle, is the dominant tree species in many Tasmanian and
Victorian cool temperate rainforest communities. The main cause of myrtle death in
undisturbed stands is the disease myrtle wilt, which is caused by the pathogenic
hyphomycete Chalara australis. Early literature, aerial surveys and aerial photography
indicated that myrtle wilt was endemic in at least part (and possibly most) of the range of
myrtle, but that in some areas, disease levels may have increased in the recent past.
In Tasmania, measured mortality rates were found to be variable but not escalating, with no
apparent overall trend. A new estimate of annual mortality due to myrtle wilt was calculated
to be 0.61 % p.a. Logging, thinning and reading of myrtle-dominated rainforest led to
increased myrtle wilt incidence. For some disturbed areas, there was evidence that after an
average of nine years, elevated myrtle wilt mortality levels declined to background levels.
The spread of myrtle wilt into areas adjacent to disturbances was clearly detectable up to
180 m from the disturbance, although not all sites were affected.
Small, experimental stem wounds on myrtle saplings provided suitable infection courts for C.
australis spores, with most infections occurring within 14 days. Functional root grafts
commonly occurred in young myrtles, and experimental inoculation, root excavation and
sectioning strongly indicated underground, tree to tree transfer of C. australis. Re-isolations
of C. australis were made from these trees, and characterisation of these isolates verified
spread via root grafts. Root grafting probably predisposes stands to epidemics, plays a
major role in the spread of myrtle wilt, and causes the clumped pattern of infected trees.
Clumping occurred on a scale of 2.5-14 m and gave rise to patches of dead and diseased
myrtles, often resulting in large gaps in the forest canopy.
Floristic studies showed that there was no distinct set of species which characterised myrtle
wilt gaps, but that there were generally more myrtle seedlings than in control forest. Data
from one site suggested that the vegetation composition in large, old gaps was reverting to
that of the surrounding forest, and that myrtle was self replacing in such gaps. The probable
long-term effects of myrtle wilt on Tasmanian myrtles were investigated using a simple
population model.
In summary, if current levels of myrtle wilt continue, it is unlikely that the disease will lead to
any permanent change in forest structure. In undisturbed forest, myrtle wilt acts primarily as
a mechanism facilitating stand rejuvenation.

Item Type: Thesis - PhD
Authors/Creators:Packham, JM
Keywords: Nothofagus, Chalara
Copyright Holders: The Author
Copyright Information:

Copyright 29/4/16 the Author - The University is continuing to endeavour to trace the copyright owner(s) and in the meantime this item has been reproduced here in good faith. We would be pleased to hear from the copyright owner(s).

Additional Information:

Thesis (Ph.D.)--University of Tasmania, 1995. Includes bibliographical references (leaves 201-222)

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