Phytophthora cinnamomi infestation — a 24-year study of vegetation change in forests and woodlands of the Grampians, Western Victoria
Authors: Weste Gretna , Brown Kenneth , Kennedy Jill , Walshe Terry 
Published: September 2002

Changes in vegetation, pathogen population and distribution were monitored
periodically in both defined infested quadrats and similar pathogen-free
quadrats at six sites representing major types of forest and woodland.
Assessments were recorded in May 1977, 1979, 1981, 1983–1984, 1995 and
2000. The susceptible eucalypts in the overstorey of infested sites, such as
Eucalyptus obliqua, E. baxteri,
E. willisii and E. macroryncha,
showed severe dieback, loss of crown or deaths. All the trees died on some
sites, others presented dead leaders with epicormic growth on lower branches.
Dieback followed by death occurred in 54% of the understorey species,
including the dominant Xanthorrhoea australis, thereby
changing community structure and species composition. At the time of its
greatest prevalence, the pathogen’s activity resulted in a decline in
species richness in infested quadrats to a mean of 25.6 species compared with
a mean of 39.2 for pathogen-free quadrats. Percentage cover and percentage
contribution to the community by susceptible species were negligible. On steep
sites, 65% of the ground remained bare, but on other sites the
susceptible flora was replaced by field-resistant species of sedges and
rushes, such as Lepidosperma semiteres and
Hypolaena fastigiata, and by partly resistant tea-trees
Leptospermum myrsinoides,
L. continentis and L. scoparium.
The dense, field-resistant understorey consisted of the ground cover of
H. fastigiata, scattered clumps of various sedges and
above this a mass of tea-tree scrub, approximately 1 m in height, with
moderately severe dieback of the branches.
From 1976 to 1984, the pathogen was isolated from 100% of the 345 root
and soil samples and from all of the infested quadrats, but then gradually
declined. In 2000, Phytophthora cinnamomi Rands was rare
at four sites and was not isolated from two sites. Regeneration of 30
susceptible species, previously eliminated, was recorded from infested sites
and 21 of these species were growing in more than one quadrat. Vigorous
regeneration of the previously dominant but highly susceptible
X. australis occurred at two sites and was similar to
that recorded from some recovering infested sites in the Brisbane Ranges,
Victoria. The decline of the pathogen and the regeneration of susceptible
species may be associated with low spring rainfall from 1995 to 2000 and the
consequent reduction in zoospore production, enabling a partial recovery from
dieback. The disease cycle from invasion and destruction of a susceptible
indigenous flora by this virulent pathogen to the decline of the pathogen and
the regeneration of that same susceptible indigenous flora was almost complete
on sections of two of the six sites studied. In other areas, the
post-infection colonising flora of field-resistant species remained dominant,
except at one steep site where the ground remained uncolonised and
subsequently eroded following the death of susceptible flora. Extinction
following infection by P. cinnamomi, however, remains a
grave threat to endangered, endemic species if susceptible.