Agriculture Reference
In-Depth Information
HUSK SPOT
■
Cause
The fungus
Pseudocercospora macadamiae
.
Symptoms
Husk spot symptoms develop very slowly, and symptoms
appear on three-quarter to full-size green fruit, six to eight
weeks after infection. Husk spot first appears as chlorotic
or yellow f lecks, which enlarge and turn tan brown in the
centre. Spots can measure 5-10 mm in diameter, coalesce,
and are surrounded by a diffuse, yellowish halo. Under
moist conditions, mature spots produce a grey velvety
carpet of spores. Husk spot is characterised by tough or
woody spots, making it easily distinguishable from diseases
such as husk rot, which is typically spongy and soft. In
addition, circular tan spots appear on the inside of the
husk. Shells and kernels are not infected by the fungus.
Depending on the macadamia variety, diseased fruits may
drop from the tree before the kernel matures. This
abscission of diseased premature fruits generally occurs
four to five months after f lowering.
Fig 11.3 Husk spot showing internal symptoms.
Source of infection and spread
The fungus produces spores that are spread by rain splash
from lesions on the surface of diseased husk tissue. Old,
diseased husks that fail to abscise are the primary source of
inoculum within the canopy, and their abundance within
the tree canopy significantly inf luences disease incidence
and severity. Old, diseased husks may continue to produce
spores for up to two years. Fruits are infected under moist
conditions from f lowering stage to maturity, but yield-
limiting infections take place the month following
f lowering when the fruits are still small. Fruit from out-of-
season f lowering (e.g. April-May) that become diseased
may provide spores for the infection of the main crop
(August-September f lowering).
Importance
Husk spot is an important disease of bearing-age trees in
south-east Queensland and northern New South Wales.
Diseased fruit may abscise prematurely, resulting in yield
loss due to immature kernel of low oil content and small
size. At harvest, if diseased, immature fruits are picked in
combination with mature fruits, the entire harvest is
commonly downgraded at the processing stage, resulting in
lower financial returns.
Management
•
Apply recommended fungicides when fruits are at match
head size on susceptible varieties.
Remove old, diseased husks in the tree canopy.
•
Fig 11.2 Husk spot showing internal and external symptoms.
