What are the most common diseases in olive trees?

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Generally, the less extreme the summer and winter temperatures, and the higher the relative humidity throughout the year, the higher the incidence of pests and diseases in the olive grove.

The olive grove on the coast tends to have many more pests and diseases than the olive grove in the interior with a continental climate.

In olive groves with hot summers, cold winters, and little humidity throughout the year, there is practically no incidence of pests and diseases, being the organic olive grove management very similar to that of the conventional olive grove.

Irrigation itself can promote the incidence of pests and diseases, when applied with the same protocols as a crop from a tropical, monsoon or Atlantic climate: not maintaining certain water stress in summer.

In the areas of origin of the olive grove, the best productive areas are not those with the most fertile soils, or even the best climate, but those with hostile temperatures and humidity for pests and diseases, but which the olive tree can tolerate.

Olive grove ESAO Image Bank

The first pillar for the control of pests and diseases in the olive grove should be to understand that the olive tree is usually much more resistant to cold, heat and dryness than its pests.

Pests and diseases in olive trees can attack the fruit, vegetative structures or both.

If the olive plague attacks only the fruit, the olive, the current harvest is affected, but not the next one.

When the olive plague attacks the vegetative structures, the harvest of the current year is compromised, as well as that of the following year.

REPILO (LEAF SPOTS)

Spilocacea oleagina (peacock spot) &

Pseudocercospora cladosporoides (Cercospora leaf spot).

These are fungi that attack the olive leaf, causing defoliation, which reduces the photosynthetic and productive capacity of the olive tree. They do not usually attack the fruit, but only the leaf.

Olive grove with repilo. ESAO Image Bank

In conditions of mild temperatures and leaf wet with static water due to dew or high relative humidity, for many hours at a time throughout the year, the spores of these fungi are capable of penetrating the cuticle of the leaf and infecting it.

The fungus develops its tissues inside the leaf, feeding on it, and producing spores outside, either on the upper side of the leaf, such as Spilocacea oleagina, or on the underside, such as Pseudocercospora cladosporoides.

In most olive groves, the ideal conditions for infection occur in spring and fall.

In climates with mild temperatures and high relative humidity, either in summer or in winter, these fungi have a very high economic impact on the olive grove.

The olive tree eliminates the leaves with very advanced infections as a means to reduce the inoculum, thus losing photosynthetic capacity, which translates into worse vegetative growth and worse flowering.

There are preventive and curative means of control.

As the first line of defense, the most logical thing is to try to avoid olive cultivation in climates that are very conducive to this disease.

Olive tree with low crown density, which makes the survival and expansion of the repilo very difficult.

Prevention for practical purposes is based on pruning the olive grove and the use of copper.

Maintaining the olive tree by pruning it with well aerated glasses contributes a lot to reducing the incidence of repilos.

Treatments with copper salts are the classic method of prevention against the repilo, since the copper ions released by these salts on the surface of the leaf affect the spores when infecting the leaf. However, they are not capable of seriously affecting already established infections.

It is about always having a copper film on the sheet, just before and at the beginning of periods of high relative humidity and mild temperatures.

It is usually treated at the end of summer and at the end of winter, although depending on the farm, these treatments can be extended or shortened.

As curative means, there are synthetic fungicides, systemic translaminar curative eradicators, capable of eliminating infections already established inside the leaf.

These treatments are applied in the middle of spring and autumn, if that were the case.

However, they are not fully effective, as they are not capable of completely healing the most advanced infections, only incipient ones, so their use must be combined with preventive means, such as copper.

Eradicating fungicides are not authorized in organic farming, being very difficult to profitably cultivate olive groves in areas with a very high incidence of repilo, as well as with high crown volumes, as there are restrictions on the amount of copper applicable in an olive grove per hectare no longer.

Leaden repilo. ESAO Image Bank

OLIVE FLY MAGGOT

Dacus oleae

This insect affects only the fruit, having no impact on the rest of the tree. In fact, it only feeds on the fruit in the larval phase, living the adult phase outside the olive grove.

Olive fly larva. ESAO Image Bank

Most of the year, the fly lives as an adult, and outside the olive grove.

The adult olive fly is very sensitive both to very high temperatures with low relative humidity, and to very low temperatures below freezing.

It feeds on flower nectar, fruit and vegetable juice that it takes from small wounds on the skin of these fruits, etc.

Therefore, environments with a coastal climate, with numerous riverside forests, with water points, and many fruit trees and vegetables, would be the ideal habitat for the olive fly.

In these contexts, olive groves are usually abandoned, due to the tremendous difficulty in obtaining healthy fruits.

In mountain olive groves, summers are usually very benign for the fly, although winters are not so, and the incidence of the pest is usually medium.

In olive groves in the countryside, with hot and dry summers, with relatively cold winters, and without much shelter or other horticultural or fruit crops in the surroundings, the incidence of the fly is usually minimal or non-existent.

The olive fly larvae develop in the olive mesocarp between summer and autumn. When leaving the fruit as an adult, it leaves a hole in the olive skin, through which fungi penetrate that rot the fruit, deteriorating the organoleptic characteristics of the oil obtained.

There are preventive and curative means to control the olive fly.

Preventive. They are applied from summer, when the female flies begin to make their first spawns. These means are the only ones authorized in organic farming:

• Diatomaceous earth or kaolin. They prevent female flies from landing on the olive tree and making their lay.
• Mass trapping. They eliminate individuals, reducing the number of females that do the laying.
• Patching. A mixture of attractant and insecticide is used, which the flies ingest and die.

Eradicating curative means: use of insecticides, either shock to eliminate adults, or systemic to kill the eggs and larvae inside the fruit.

Detail of fruit in which the fly larvae have already developed and abandoned, leaving an exit hole through which moisture and fungi will penetrate, which rot the olive inside.

TUBERCULOSIS

Pseudomonas savastonoi

(olive knot)

Olive tree with tuberculosis. ESAO Image Bank

It consists of a bacterium that produces small tumors on the thin branches, which greatly depress the vigor and production of these shoots and of the tree in general.

Its incidence has mainly a varietal component, although the climate and especially the management also influence.

Through small wounds in the bark of young branches, when the bark has static liquid water, and with mild temperatures, bacteria of this species settle, founding new nodules.

Therefore, conditions with abundant and frequent wounds in the bark of young branches, with high relative humidity and mild temperatures, and sensitive varieties, are the scenario that makes this disease can be found in an olive grove, being able to have a great economic importance.

Injuries to the bark are usually caused by harvesting, hail, or severe frost.

Snowy olive grove. ESAO Image Bank

As prevention, you should have canopies with low vegetative density, so that harvesting or the wind itself does not cause so many injuries and friction.

In pruning, all severely affected twigs must be thoroughly removed, as they are the source of inoculum for new infections.

As a curative treatment, there is only copper, which prevents the installation of new tumors.

There are no effective curative treatments for this disease. Preventive measures are the same for organic olive groves as for conventional ones.

Olive tree tuberculosis. The scarce vigor of the affected branches can be observed.

ANTRHACNOSE AND OLIVE LEPROSY

Colletotrichum acutatum/gloeosporides (anthracnose)

Phlyctema vagabunda (olive leprosy)

They are different fungi, although with very similar features. They affect both the fruit and the shoots, affecting both the current harvest and the following year or even longer term.

The fruits rot while still attached to the tree itself, mainly from January.

Ripe olives on the tree, infected by anthracnose, the main source of dispersal of the inoculum

Olives rotten by anthracnose. ESAO Image Bank

The infected fruits disperse the spores throughout the rest of the crown, also affecting young branches, which become dry.

The affected fruits inevitably lose their quality and give rise to defective oils.

The affected dry shoots do not produce in the following seasons.

Detail of leaves affected by leprosy

For this disease, there are both varietal and climatic components to explain its incidence, although also the management of the olive tree itself.

Early harvests before veraisoning, whether for table olives or for premium oils, prevent the fungus from closing its life cycle.

In the case of leprosy, very aggressive harvests in cold weather, in olive trees with very dense crowns, which cause frequent injuries to the bark of young branches, greatly favor the incidence of this disease.

For anthracnose, preventive measures would be early harvesting and the use of copper treatments as early as summer.

For leprosy, prevention would consist of removing affected branches and maintaining low crown densities at harvest, but copper treatments do not appear to be effective. With curative systemic fungicides, it does seem that there is control, but this tool is not allowed in organic farming.

Defoliation and branch death caused by anthracnose in an Australian olive grove.

Olive tree very affected by leprosy

These agents are the most common and most economically important in the olive grove, although occasionally other agents can be very harmful, depending on the farm. In the Olive Grove Management module within the Master Olive Oil Consultant Certification you can learn from technicians and professionals with extensive experience in the field of olive grove management. Learn from the mistakes in the care of the olive grove, being able to address all the doubts and concerns that may arise, understand what are the key factors that influence the behavior of the olive tree, acquire first-hand knowledge of the best practices when managing the olive grove and get in touch with other professionals in the olive sector.