Olive leprosy or anthracnose is a severe fungal disease that affects olive trees. It has two scientific names: Colletotrichum gloesporioides and Gloeosporium olivarum. It is challenging to control, especially because the damage becomes evident around the time of olive harvesting. The olive leprosy has been known since the 1950s, particularly in olive groves in the South. After a period in which it seemed to have disappeared, this disease has become concerning again in recent years, affecting olive groves in various regions of Italy where intensive olive cultivation is present.
Let’s see how to identify olives affected by leprosy and what damages it causes. Additionally, we will explore the techniques of agronomic prevention and the biological treatments to be applied.
The pathogen Colletotrichum gloesporioides
The pathogen Colletotrichum gloesporioides appears in numerous genetically distinct strains. They are all difficult to isolate precisely, are mutable, and constantly evolving. For this reason, olive leprosy is a disease that greatly concerns olive growers. The fungus causing it infiltrates infected fruits that fall to the ground and rot. It also penetrates seeds and plant residues. Furthermore, the pathogen can also enter plant tissues, thanks to the fungus’s conidia. These give rise to a germ tube with an appressorium that spreads the disease through natural openings (stomata and/or lenticels), microlesions, or wounds of any nature. The spread of anthracnose occurs at the beginning of autumn and is favored by heavy rain with temperatures around 15 °C. The optimal temperature for the pathogen’s development is between 20 and 25 °C. In general, mild and humid years favor its spread. On the contrary, very cold winters followed by hot and dry summers are environmental factors that limit the disease. Furthermore, the olive fruit fly can also be a dangerous vector of leprosy.
Symptoms and Damages of Olive Leprosy
Olive leprosy causes damage not only to the fruits, but also to leaves, twigs, small branches, and more rarely, flowers.
Damages of leprosy on olives
On olives, usually during the veraison period, dark patches with a depressed area form due to leprosy. The spot expands and can cover the entire drupe, which dries up and undergoes premature drop. This results in significant damage to oil production. Furthermore, olives that are not dropped but still infected yield low-quality oil. The oil obtained from olives affected by leprosy has an acidic reaction. Additionally, it is cloudy, more turbid, and tends towards reddish.
Damages of leprosy on other parts
As mentioned, the infection of leprosy can also affect olive leaves. These develop chlorotic and yellowish spots with an undefined border. The spot tends to widen (turning brown) until it reaches the edges of the leaf lamina. The end result is that the leaf dries up and falls off. On twigs and small branches, olive leprosy forms dry, whitish, round, and slightly irregular spots. Here too, the affected parts progressively wither. The risk on infected plants is that a dangerous vicious circle is initiated. Sometimes, affected fruits do not fall to the ground but mummify on the tree, becoming the inoculum for the next season. The fungus then passes from the fruit peduncle to twigs and leaves. The following year, it moves to newly attached flowers and drupes. Here, the leprosy remains latent until veraison, when the olives start to mature.
Preventing Olive Leprosy
To prevent olive leprosy, it is essential to implement specific agronomic techniques to prevent its spread.
Ground Covering in the Olive Grove
First and foremost, soil preparation work should be carried out to ensure excellent water drainage. This consideration applies to all horticultural crops. In this sense, olive cultivation managed using ground covering techniques can help, especially for flat terrain.
Pruning Olive Trees
Pruning olive trees is very important, with targeted cuts to thin out and ventilate the canopy. By doing so, conditions of humidity that favor the spread of the olive leprosy pathogen are avoided. It is also necessary to remove all parts affected by the disease. The procedure is similar to that done for another pathology, namely: olive scab. When working on infected plants, it is important to disinfect pruning tools before moving on to another plant. The risk is to transfer the disease to healthy plants or increase the inoculum level in already infected ones. Finally, it is crucial to remove all vegetative residues that could retain the pathogen’s inoculum. This includes both olives that have fallen to the ground and are destined to rot and those that have not been harvested and mummify on the tree. It is there that anthracnose hides.
Defending Olive Trees from Leprosy
Biological defense against olive leprosy, as with other fungal diseases like tomato downy mildew, is carried out using copper salts. An example is Bordeaux mixture, which you can also try making yourself. Alternatively, preparations based on sulfur can be used. These are products allowed in organic agriculture. The best time for these treatments on olive trees is during the first rains of late summer and early autumn. We are talking about the beginning of veraison, with the treatment repeated at the end of October or before the olive harvest, considering the waiting times. Treatment with copper-based products is justified only on olive groves that have been attacked by olive leprosy in previous seasons. This refers to plants with a high potential for inoculum. Also, climatic conditions must pose a risk; it’s pointless to treat in the absence of precipitation. Another consideration to make is that if you are treating your olive grove for peacock spot, which is the most critical cryptogamic disease, the interventions should also suffice for leprosy containment. Therefore, carefully evaluate the state of your olive grove before anything else.