Perhaps not everyone knows that the olive tree can be fiefdom (inhabited) by over 250 species, which include mushrooms, insects, mites and bacteria. Of these 250 species, only some are harmful and can cause damage to the plant and its fruit.
Olive tree diseases can be divided into two macro categories: those caused by insects and parasites, and those caused by bacteria and fungi.
But let’s get to know them together
DISEASES CAUSED BY PARASITES AND INSECTS
Olive fly (bactrocera oleae)
The distribution of the olive fly coincides with that of all areas where the olive tree is grown in the Mediterranean; widespread on maritime coasts, it can also be found in inland areas, where its harmfulness is however reduced.
In Italy it is present in all olive growing areas, and is the most present parasite.
The adult insect is small in size, 5 mm, and lays its eggs inside the olive, feeding on it.
The degree of ripeness and the color of the foliage are two important factors for the fly, the green-yellow or reddish color is the favorite color for the female, while for the adult male it is yellow, especially if fluorescent.
Its presence in the different stages is as follows: adults are present all year round in the olive grove; the eggs are had from July until the temperature drops to 13 degrees, therefore tends to be in October; the larvae are present from July until the end of November.
The fly settles inside the drupe and can cause: the erosion of the pulp due to the larvae, the fall (in technical jargon falls) of the infested olive, alteration of the quality of the olive. The fall of the olives is the main damage that the fly can cause, because it makes the olive properly unusable.
To combat the oil fly, in recent years a type of trap has been developed containing a multitude of possibilities, namely fluorescent yellow color, ammonia dispenser and sex pheromone dispenser.
Olive thrips (Liothrips oleae)
The olive thrips is an insect found mainly in mountain and hilly areas throughout the Mediterranean basin.
This insect is smaller than the oil fly and feeds itself by sucking the sap from the more tender olive leaves, as well as from its shoots. The damage caused by thrips is present in most olive groves, but never reaches significant economic dimensions.
There is no need to intervene with specific treatments.
Cotonello dell’olivo (Euphillura olivina)
The olive cotonello is an insect widespread in all olive growing areas of the Mediterranean area, mainly in coastal areas with high humidity.
This greenish insect causes minor damage; on the infested plant organs it creates abundant waxy secretions with a similar appearance to cotton, from which it takes its name; only in late spring do we find, by this insect, more intense damage associated with the infestations that occur in the moments of flowering and fruit set.
To combat this insect, it is important to ventilate the foliage in order to reduce humidity and its consequent proliferation.
Cochineal (various species)
Cochineal is a widespread parasite of the olive tree. There are different types, many of which are not harmful, or in any case create minor damage.
When these insects settle on the olive tree, they prefer the cracks in the leaves, settle in small colonies, and are able to occupy the entire leaf structure of the plant, feeding on its sap.
The cochineal looks for places that tend to be far from the sun’s rays and in poorly ventilated and dry environments; its reproduction takes place in the summer months, as it loves to reproduce in the heat.
The classic alarm bells are: yellowing or wrinkling of the foliage, or the appearance of spots.
The most harmful cochineal is the “half peppercorn”, which settles in plants and damages them by subtracting sap destined for the shoots, which consequently dry up or perish
For the other varieties of Cochineal, chemical interventions are usually not necessary because the actions of the natural antagonistic agents are sufficient.
Olive moth (Prays oleae)
The olive moth is a small moth, the adult is a butterfly measuring 14 mm, which generally does not cause serious damage, although it can sometimes cause a substantial drop of the olive fruit.
It is a species that completes three generations a year: the first generation larva damages the inflorescence, the chrysalis damages the drupe and the butterfly erodes the leaf blade, the shoots and the olives.
There are no preventive measures that can control this insect.
Green olive moth or Margaronia (Palpita unionalis)
Margaronia is a moth, with a wingspan of 25 mm, particularly feared in young olive groves.
It creates damage by eroding the tips of the growing plant, thus also hindering the growth of leaves and shoots and the form of cultivation of the chosen plant. In the event of major attacks by this insect, it can even stop the development of the plant.
Particularly harmful are the late attacks, after mid-September, which cause the slowdown of the vegetative restart.
Normally Margaronia attacks do not justify chemical interventions. In addition, the insect is kept away from the olive grove by the treatments performed against the oil fly.
DISEASES CAUSED BY MUSHROOMS AND BACTERIA
Peacock’s eye (Spilocaea oleaginea)
Peacock’s eye, also known as Cycloconio, is the most important fungal disease of the olive tree.
The disease is widespread throughout the olive growing areas of the Mediterranean and tends to appear in spring.
This disease develops in very humid environments and temperatures between 16 ° and 24 °.
Its spread occurs through the conidia, but above all through the precipitation waters, in fact the settlement of this disease almost always occurs after prolonged periods of rain.
The infections affect all the green organs, therefore the leaves but also the young branches, causing notable defoliation; the fall of the leaves can also compromise the life of the plant itself in case of serious infestations. On the leaves affected by the peacock eye, there are characteristic circular spots surrounded by a halo that in the summer turns yellow, reminiscent of a peacock, a species from which it takes its name.
To combat this disease, it is advisable to: adopt not too dense planting layouts, to facilitate air circulation and the access of light to the olive grove; adopt balanced fertilizations and improve the channeling of rainwater to avoid stagnation within the olive grove.
The preventive defense is carried out with copper-based products to be carried out in spring and autumn.
Cercosporiosis of the olive tree or Olive plummet (Mycocentrospora cladosporioides)
This disease mainly affects the foliar apparatus of the olive tree, although in recent years it has also affected the fruit.
As the “Peacock’s eye” is widespread in the countries of the Mediterranean basin, it presents a development linked to the meteorological trend and can cause important defoliation and consequent negative repercussions on the productivity of the plant.
Infections occur in autumn and spring, spreading through the conidia by the wind or by the rains that bring the fungus to the plant.
The fungus can attack the twigs, the peduncles, the leaf petioles and the drupes; when the fungus invades the entire leaf, it causes yellowing.
Copper-based products normally used for peacock eye disease help to keep this disease under control as well.
Verticilliosis (Verticillium dahliae)
Verticilliosis is a fungus present in the soil, capable of attacking the olive tree at the vascular level.
Present in all the countries of the Mediterranean basin, this fungus penetrates the tissues of the plant, introduces itself into the vessels and destroys the walls with its toxins, preventing the natural flow of water.
Typical disease of new plants, even if also found in old olive groves, occurs on single plants and then creates a stain inside the olive grove. In spring, the first symptoms consisting in the discoloration of the leaves are noted, subsequently these dry up and fall, even if they remain dry for a long time, attached to the branch.
For the defense against this disease, only preventive agronomic interventions are found at this time, such as the removal and burning of dried branches.
Olive leprosy (Colletotrichum gloesporioides)
The leprosy of the olive tree takes its name from the typical alterations it causes to the drupe of the olive tree, with the “mummification” and “rot” of the fruit.
With this disease, drying and falling off of twigs and inflorescences can also occur.
It is extremely widespread, and finds its ideal climate in mild and humid temperatures.
The fruit drops early with serious damage to the plant’s production level. Infected olives will produce a poor oil with high acidity.
In addition to preventive agronomic interventions, such as good pruning, thinning of the foliage, etc., it is possible to consider treatments with copper, which are also allowed in organic cultivation.
Massariella (Massariella oleae)
On the olive tree, the Massariella mushroom, favored by poor ventilation and high humidity conditions, gives rise to small dark areas that swell. Symptoms are borne by the branches and suckers. The branch takes on a gnarled appearance, with dark concentric halos.
This type of disease mainly affects unmanaged or abandoned olive groves and in particularly humid and not very sunny areas.
Normally, specific interventions are not necessary to defend against this pathology.
This term indicates a “darkening” of a fungal nature that covers the surface of the leaves of the plants.
These fungi derive their sustenance from the sugars present on the leaf.
The persistence of this disease on the plant can cause defoliation and poor fruiting.
Chemical interventions are not recommended against fumaggine, but a preventive agronomic defense that ensures that the plant is ventilated and well pruned.
Olive tree mange (Pseudomonas syringae pv. Savastanoi)
Olive mange is a disease widespread worldwide wherever the olive tree is grown, caused by a bacterium that enters the woody tissues of the tree, infecting it.
The bacterium penetrates following wounds or scars originating at the meteorological or pruning level.
The characteristic tubercles appear on the plant after an incubation period that can vary from 1 to 3 months.
The largest mange outbreaks occur soon after major hailstorms or late colds.
Infections are favored by mild climates and high humidity.
Also in this case, the defense against mange is to implement preventive agronomic interventions.
These just listed are some diseases that affect olive trees.
Alberti’s advice is to thoroughly study your olive grove site, the most present cultivar, the climate and the surrounding area so as to be able to act preventively and not be caught unprepared by the attack of these species