Anthracnose in Olive Trees

How to recognize and manage anthracnose in olive trees

Olive anthracnose is the most important disease of the olive fruit. The disease is fungal and caused by several species of the genus Colletotrichum. This genus consists of deuteromycota (anamorph), which rarely show the perfect form (teleomorph) of Glomerella in nature, a genus that belongs to the ascomycota. The predominant pathogens responsible for causing the disease, after many changes in the systematic taxonomy of the species, are considered to be the species C. acutatum s.l. and C. gloeosporioides s.l.. C. acutatum s.l. is a complex of species and different morphotypes.

anthracnose in olive trees

Photo by Stefanos Kolainis

Infection of the fruit leads to quantity and quality losses in production. In the case of an epidemic, the produced olive oil is of low quality, high acidity, and reddish color, making it unfit for human consumption, according to the regulation of the European Community (E.C. No 1989/2003). Moreover, the pathogen causes extensive quality loss of the olives meant for table use. The olive fruit, affected by anthracnose, initially shows a reddish-brown spot that spreads on the fruit surface. The infected tissues wrinkle, and in high humidity conditions, orange fungal spore masses (conidia) may be observed to cover the spot. The fruits then fall to the ground and rot or shrink, mummify, and remain attached to the tree. These mummified fruits are the source of infection for next spring.

The growth of the fungus seems to be optimum at a temperature of 10-25 °C (ideal 25 °C). On the contrary, the growth stops at 0 °C and is minimal at 29 °C. As a result, it becomes apparent that the optimum temperatures for fungus growth are usually recorded during the spring, autumn, and the cool days of summer. When these conditions are combined with high relative humidity and rainfall, the infection spreads and can lead to an epidemic outbreak in a region. The disease's life cycle begins in the spring when the temperature rises. The conidia are released from the acervuli found in the previous year's fruit and infect flowers, immature fruits, and rarely leaves, branches, and fruit stems. However, latent infection of the flowers without the appearance of symptoms has been observed during fruit ripening. During summer, when the temperature is relatively low and the humidity high (with a cloudy sky), the disease develops on the unripe fruits. Later in autumn and winter, the fruits that are near maturation become the most vulnerable to infection.

Anthracnose in olive trees

It's worth mentioning that the infestation of olive-fruit fly (Bactrocera oleae) contributes to the development of the disease, as the fruit with punctures matures faster. However, the insect is not considered a necessary factor for the infection.

Control of anthracnose includes both cultivation and chemical control measures. The purpose of the cultivation measures is to reduce the humidity. This is mainly achieved by planting the trees at a higher distance and pruning for better aeration of the canopy. If the farmer starts an olive orchard from the start, selecting a field in a region with lower humidity levels is best.

In addition, early harvesting, which offers high-quality olive oil, is also a key factor in the control of the pathogen. The use of Chemical Plant Protection Products (PPPs), along with cultivation measures, is necessary in most cases. The international bibliography reports that control can be achieved by using copper, dithiocarbamates, phthalimides, triazoles, strobilurins, benzimidazoles, chlorothalonil, and fosetyl-Al. Always use fungicides suitable for the crop, certified, and allowed in your country. A common practice was to spray during autumn with copper-based PPPs. In recent years, some complementary applications have been advised to be performed during spring for a more complete plant protection. The combination of active compounds (e.g., triazole - strobilurins) is generally recommended to avoid and manage the pathogen's resistance to these products. PPPs with essential oils (i.e., terpenes) are also considered effective against these pathogens.

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