440 
tabliehen Stadium begrundigt sich auf Symptome in Einzel 
pflanzen im dem Unterwuchs. Zuverlassliche Entdeckung der 
Krankheitssymptome kann mann durch grossmasstäbliche Photo 
graphien erreichen, um die Zuverlässlichkeit des kleinmass- 
täblichen Stadiums zu verbessern bedarf es dagegen ferner 
Forschung. 
INTRODUCTION 
The jarrah ( Eucalyptus marginata Sm) forests of 
western Australia are severely affected by "jarrah dieback", 
a disease caused by the root rotting pathogen Phytophthora 
cinnamomi Rands (Podger, Doepel and Zentmyer 196> 5)• First 
reported in 1921 in small areas of the northern jarrah 
forest, the disease now assumes major proportions with an 
estimated 170,000 ha of forest affected by the disease 
(Forests Department 1974)• Reliable mapping of the disease 
is necessary for overall resource planning, scheduling 
salvage operations and for the forest hygiene program aimed 
at reducing the spread of the disease (Batini and Hopkins 
1 972). 
The symptoms of the disease are decline and death 
of the jarrah as well as a wide range of under storey and 
shrub species (Podger 1972). The classical manifestation 
of the disease is the yellowing and death of a number of 
understorey and shrub species, the most obvious ones being 
bull banksia ( Banksia grandis Willd.) . zamia palm ( Macrozamia 
re idle i O.A. G-ardn.) and blackboy (Xa nthorrhoea preissii 
Endl.) , tliough the latter may frequently survive even in 
advanced dieback areas. Obvious deterioration of the jarrah 
crowns is usually not apparent until some time after symptoms 
appear in the understorey, but ultimately the jarrah becomes * 
thin crowned, and the tree eventually dies. The smaller 
and less obvious susceptible shrub species also die out 
during this process leaving a stand of dead jarrah with a 
light shrub cover. 
Marri ( E. calophylla R.Br.) . the other main tree 
species of the jarrah forest association is resistant to 
P. cinnamomi and together with other resistant shrub species 
eventually recolonises the site. A transect through a 
classical dieback site exhibits all stages of the disease 
from the first deaths of the banksia through to stands of 
affected and dead jarrah with live marri. Variations from 
this pattern are not uncommon due to varying degrees of sus 
ceptibility, rate of fungal activity and to the variable 
proportions of susceptible and resistant species in the 
understorey. These variations sometimes cause difficulty 
in deciding whether or not an area is affected by P.cinnamomi . 
In the northern jarrah forest region (Figure 1), 
the mapping of dieback from 1: 40,000 panchromatic aerial 
photographs has been successful for detection of the disease 
at about the stage where the jarrah itself is being affected.