PESTS AND DISEASES OF FORESTRY IN NEW ZEALAND
MAPPING SEPTORIA LEAF BLIGHT
Back to septoria leaf blight
Mapping septoria leaf blight
From Forest Health News 164, July 2006
In parts of New Zealand young plantations of Eucalyptus nitens
are affected by a disease known as septoria leaf blight. Juvenile
leaves, in particular, are disfigured by yellow spots that turn bright
crimson and eventually brown, before being cast prematurely later
in the same season. These symptoms are caused by the fungus
Phaeophleospora eucalypti (formerly known as Septoria ).
Although this pathogen was first found in New Zealand in 1981,
the disease did not become important until blocks of E. nitens
were established more widely during the 1990s. Research revealed
that the disease builds up epidemically during spring and summer
in the worst-affected stands. Control by aerial spraying with
fungicide appears unpromising and preliminary results from a field
trial suggest that it may be better to plant a more resistant eucalypt
species on disease-prone sites than to persevere with E. nitens . To
help identify these risk areas, a survey was conducted in the Bay
of Plenty and central North Island regions.
Altogether 49, 8- to 12-year-old blocks of E.nitens were inspected
and scored for P. eucalypti infection and for health condition.
Stands were rated as unhealthy (score 1.0) if many trees bore only
juvenile and epicormic foliage, which was moderately or heavily
infected by P. eucalypti . If stands had deep, dense, adult crowns
with mature foliage on most trees, and infection was still medium
or high on the less common juvenile and epicormic foliage, they
were classed as intermediate (2.0). When unhealthy stands were
present within predominantly intermediate category blocks, the
score was adjusted accordingly. Healthy blocks (3.0) were those
that resembled intermediate stands, but infection was absent. Maps
were prepared from GIS data in order to relate health and infection
to regional climate.
E.nitens, unhealthy, Type 1 stand
Phaeophleospora eucalypti infection was found to be widespread,
especially on juvenile and epicormic foliage, indicating that
rainfall and temperatures are suitable for the fungus to infect and
develop throughout the survey region. On the other hand, stand
health appeared to depend on temperature. Unhealthy Type 1
blocks were prevalent in a warmer climate zone near the coast,
whereas the healthier, predominantly Type 2 stands were more
typical of the cooler sites at higher elevations further inland. No
healthy Type 3 blocks were found during the survey, though they
are the norm in the southern South Island.
Since P. eucalypti is considered to be the cause of this disease,
why is stand health poorer near the coast, even though the fungus
is still common in the colder inland sites? The coastal climate is
milder than in the native range of E. nitens in continental Australia,
so these warmer conditions may prolong the susceptible juvenile
state for a longer period. In addition, the fungus may be more
damaging at these higher temperatures. The severe casting of the
diseased juvenile leaves, and the production of epicormic recovery
shoots, may also in some way check the normal switch from the
juvenile to the adult form on many trees. Conversely, the natural
change to mature foliage observed in the inland blocks appears
beneficial, since the reduced proportion of infected juvenile and
epicormic leaves will lead to lower spore levels.
The results from this survey suggest that E. nitens from Victoria
should not be cultivated within a zone 20 km from the coast in the
Bay of Plenty region. Further inland it should be established only
on slopes, avoiding ridge tops or valley floors, especially those
on a south-facing aspect. These recommendations are contingent
on findings from growth measurement data which would provide
a surer basis for determining the productivity of E. nitens and
deciding the merits of planting it in this region. For further details,
see NZ Tree Grower 27(3): 36-37, 39 (2006).
(Ian Hood & Neil Alexander, Ensis)
