During
a summer long past, the casual inspection of a Kalamata tree in
a home garden suggested a shortage of water at the roots
of the tree. The shorter than normal distance between one set
of buds and the next on the young twigs, showed that for some
reason the tree growth had been slow.
The
tree showed a poor fruit set which still pointed to a shortage
of water, at least in the winter when the trees internal preparation
for spring flowering was occurring. However, the owner of the
tree insisted that it had received plenty of water throughout
the year. Didn't this man understand how much water was enough,
or was there another problem? A later inspection started to give
some answers.
The
tree had almost completely defoliated (lost its leaves) by winter,
and in the spring, brand new healthy leaves were shooting
vigorously. Why had an evergreen olive tree lost its leaves?
Fortunately, there were still enough of the old leaves on
the tree and on the ground to answer the question.
It
was accurately concluded that the tree had been suffering from
attacks by a problem commonly known as Olive Leaf Spot or Peacock
Spot (Cycloconium oleaginum or Spilocea oleaginea).
Fungal infection by Peacock Spot had caused the leaves to drop.
A drastic reduction in leaves each year meant several months
of reduced photosynthesis which resulted in poor twig growth and
poor fruit set. So shortage of water was not the culprit.
Sooty
blotches are first seen on the leaves in winter. These blotches
develop into greenish-black circular spots that measure
up to 6 mm in diameter. There may be a faint yellow halo
around the spot. The lower branches and south side of the tree
will be more susceptible than the upper sections. This is believed
to be due to the fungal spores developing faster in shaded,
wet and cool conditions as happens lower on the tree and on the
south side away from the sun.
Infection
is normally associated with high humidity (eg rainfall) and winter
conditions (cool and low light). High temperatures restrict spore
germination and growth, making the disease inactive during summer.
One
or more large round spots will be seen on a leaf and the spots
will sometimes merge into each other. Most of the infected
leaves will fall prematurely by summer. The small number
of diseased leaves that remain on the tree during summer will
become crusty and whitish and with the cooling of the weather
in autumn, a new crop of spores are produced and spread through
the tree's foliage.
To
control the disease, infected trees should be thoroughly sprayed
with a copper containing fungicide in late autumn. (IMPORTANT:
See "Treatment" below) If the problem is severe, then
another application may be needed in early winter. This treatment
will often eradicate the problem completely. Your agricultural
chemical wholesaler will stock a suitable copper fungicide and
application rates should be carried out according to the label's
recommendation.
The
following excerpt comes from "Olives - Pest Management Guidelines"
(UCPMG Publication 8, 1994). These guidelines cover most
possible olive problems found in Australia and California
and are available free at http://www.ipm.ucdavis.edu/PMG/selectnewpest.olives.html
if you would like a copy. (The information comes from California
so all references to places, seasons, months and treatments are
Californian).
PATHOGEN:
Spilocea oleaginea
SYMPTOMS
AND DAMAGE: Sooty blotches on leaves develop into green black
circular spots 0.1 to 0.5 inch (2.5 to 12.5 mm) in diameter. There
may be a faint yellow halo around the spot. More lesions develop
low in the tree. Leaves fall prematurely and twig death may occur
due to defoliation.
COMMENTS
ON THE DISEASE: The fungus survives on trees in old leaf lesions
that have a white, crusty appearance. The margins of these lesions
enlarge in fall (autumn) and a new crop of spores develops there.
Infection is associated with rainfall; most infections occur
during the winter. High temperatures restrict spore germination
and growth, thus the disease is inactive during the warm, dry
summers in California.
WHEN
TO TREAT: Apply in late October (late April in Australia)
before winter rains begin.
TREATMENT:
[Due to the chemical nature of the treatments, Olive Agencies
is unable to recommend dosages or chemicals to be used. Please
check with your agricultural department and agricultural
chemical supplier as to the suitability to olives, method of application
and safety precautions needed for copper based fungal sprays.
Californian olive growers use Copper sprays.]
