The Cercospora Leaf Spot of Olive is a severe fungal disease that affects olive groves. This ailment has been known since the late 1800s and has sporadically affected different areas with olive trees. In recent years, there has been an increased presence of cercospora leaf spot, with new areas affected by the disease and a concerning rise in damages.
In this article, we will delve into the biology of the fungus, the damages it causes, and how to employ biological defense measures.
The Damages Caused by Cercospora Leaf Spot
Cercospora leaf spot of olive is caused by the fungus Pseudocercospora cladosporioides (or Mycocentrospora cladosporioides). It primarily affects the leaves of the olive tree, although with increasing pathogen pressure, damages to the fruit are also observed.
Different symptoms manifest on the leaf surface, varying between the upper and lower sides:
- On the lower side, irregular spots appear, from which a thin layer of conidiophores caused by the pathogen emerges. The coloration turns gray, hence the common name “sooty mold of olive”.
- On the upper side, corresponding to the underlying spots, initially yellow (chlorotic) and then necrotic spots appear. As the disease progresses during the spring season, the leaves eventually fall off.
- Young branches may exhibit irregularly shaped, grayish spots.
- On olives affected by cercospora leaf spot, small lesions with diameters up to 1 cm become apparent. These lesions appear as depressed or sunken areas, brown in color, with a yellowish halo. Depending on the cultivar, the lesions on the fruit vary in intensity.
Damages on Olive Production
If the degree of olive cercospora leaf spot infestation is high, the tree canopy may experience significant defoliation. The loss of leaves inevitably affects production, resulting in significant losses for olive growers.
When the pathogen attacks the fruit, premature fruit drop occurs, typically before the olive harvest. Consequently, these olives fall off without giving the olive grower a chance to intervene. For table olives, this damage is severe because, even if they don’t fall, they are no longer marketable. In the case of oil olives that are still harvested, this pathology causes increased oil oxidation levels, although it does not affect the total acidity.
How the Pathogen of Olive Cercospora Leaf Spot Acts
The pathogen of cercospora leaf spot, Mycocentrospora cladosporioides, develops on the leaves. Once it has developed, it can penetrate the cuticle through small injuries or via natural openings (stomata).
Following penetration, the symptoms described above manifest. The fungus produces conidia that enable its spread. It also produces sclerotia, which are conservative structures that allow the pathogen to survive in unfavorable conditions.
Climatic Conditions Favorable for the Spread of Olive Cercospora Leaf Spot
The best climatic conditions for the formation and spread of conidia are those with temperatures ranging between 12 °C and 28 °C and high air humidity, typically in spring and autumn.
Harsh winters and hot, dry summers naturally limit the spread of cercospora leaf spot. During spring and autumn, wind and precipitation facilitate the transport of fungal reproductive organs from one plant to another. As the infestation progresses in the tree canopy, precious cellular material is lost from the leaf tissue, leading to the appearance of spots on the lower surface and chlorosis on the upper surface.
Preventing Olive Cercospora Leaf Spot
Proper olive cultivation forms the basis of agronomic prevention of cercospora leaf spot. Sooty mold of olive can be limited by employing the following techniques:
- Balanced olive pruning
- Cleaning the field from pruning debris and natural leaf and fruit casings
- Light nitrogen fertilization
- Proper soil cultivation and/or weed management.
Olive Varieties Resistant to Olive Cercospora Leaf Spot
There are olive varieties that have shown greater resistance to cercospora leaf spot, while others are more susceptible. This information is crucial, especially when establishing new olive groves in areas where the disease is prevalent.
The more resistant varieties include:
- Tonda Iblea,
- Nocellara Etnea,
- Leccino (also resistant to xylella fastidiosa).
On the other hand, the more susceptible varieties are:
- Nocellara pugliese
Defending Olive Trees Against Cercospora Leaf Spot of Olive
Cercospora Leaf Spot of Olive presents symptoms and temporal analogies with another well-known disease in olive groves, namely, peacock’s eye (or cycloconium). For this reason, it has been assumed that treatments for the latter disease are also sufficient for this pathology.
In organic farming, the prevention Cercospora Leaf Spot of Olive primarily involves the use of copper salts, specifically cuprous oxychloride. These are allowed products in organic farming, within certain usage limits.
Copper treatments are typically applied in the autumn, immediately after harvest, or at the beginning of spring, following pruning operations. In cases of severe infestations, up to four treatments may be required. It’s advisable to avoid treatments during the summer to prevent burns and phytotoxicity.
A degree of effectiveness against olive cercospora leaf spot can be achieved by applying kaolin during the summer. Kaolin is primarily used in combating olive fruit fly, but its action is also beneficial against fungal diseases.
For kaolin application, the opposite of copper salts holds true, as kaolin provides protection against sunburn, is non-toxic, and washes off easily. As we’ve seen, the risk of this pathology of olive is high pre-harvest, making kaolin use particularly relevant during periods of increased infection risk. Here are specific packages of kaolin for organic farming.