What can happen in an olive grove that can contribute to the signs and symptoms of Peacock Spot?
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 olive tree showed a poor fruit set which still pointed to a shortage of water, at least in the winter when the tree’s 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 the 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.
Peacock Spot Explained by Expert Olive Consultant, Marcelo Berlanda
PATHOGEN: Spilocea oleaginea
Many growers now are finding that peacock spot is more apparent because of the very humid conditions which have helped spreading of this fungal disease.
Cause: This is a disease caused by a fungus called Spilocaea oleaginea
Favorable Weather Conditions: Peacock Spot thrives in relative humidity higher than 60%, temperature between 5 to 25 degrees centigrade, and Rain/Fog/Dew.
To reproduce and spread the peacock spot disease needs the above conditions and as it can be seeing this year most olive growing areas have been affected by those conditions.
All olive varieties are susceptible to this disease however the degree of infection varies among them.
Symptoms: circular spots or sooty blotches on leaves develop (usually 2mm to 10mm in diameter), at the beginning not clearly visible, but once the fungus has grown the spots are clear and usually dark in color surrounded by pale yellow tissue. As the disease spreads through the leaves it causes olive leaf defoliation. Many infected leaves remain on the tree and act as a source of spores for later infections. More lesions develop low in the olive tree.
Comments on the disease: The Peacock Spot fungus survives in old leaf lesions that have a white, crusty appearance. The margins of these legions enlarge in autumn (fall) and a new crop of spores develops there. Infection is associated with rainfall and high moisture levels; most infections occur during winter. High temperatures restrict spore germination and growth, thus the disease is less likely to be active during warm, dry summers.
Control: the best tool against Peacock Spot disease is prevention through copper sprays. A minimum of about 4 sprays are recommended per year as a preventative measure against Peacock Spot. It is recommended to start spraying as soon as the weather conditions are favorable for the disease. The first application must be followed up by successive sprays 10 to 14 days until the unfavorable conditions have disappeared.
As copper is a contact fungicide it is very important to achieve good coverage of the leaves as the untreated areas will still be susceptible to the fungus.
It is important to check the weather conditions and spray as soon as these conditions are expected.
In cases of severe infection, spraying copper will cause a leaf drop, however, there is no reason to stop spraying as this will help to reduce the number of infected leaves that remain on the tree, therefore it will reduce the number of spores on the trees.
NOTE: If the tree is flowering (i.e. with the olive flowers ready to open) it is important to avoid spraying as this could reduce fruit set and may cause the small fruits to drop.
Prevention: Once Peacock Spot disease has become endemic in the grove, it is necessary to maintain a regular copper spray as a preventative. The nutritional condition, particularly Calcium deficiencies are factors that can make the trees more susceptible to fungal infection. Stress caused by a lack of irrigation or fertilizer application is an important factor as well.
It is therefore very important to take leaf samples every summer and also monitor the soil moisture on a weekly basis.
Pruning is also a preventative tool against Peacock Spot in cases where high infections have been identified. It is important to prune your olive tree and open the canopy to allow for better airflow.
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