The threat of very high temperatures for fruit setting in olive trees in Greece

Photo by Andreas Smaragdis

Every year during this season (April-May), olive trees in Greece are at various flowering stages. Stage differs amongst cultivars and the sites of cultivation. However, the most sensitive stage for olive production is full bloom, followed by fruit setting. This is because a shortage of water and high temperature can cause serious problems for flowers, and reduce fruit setting and production. For instance, a shortage of water at this stage can cause a reduction of the number inflorescences or the number of flowers per inflorescence, or even a reduction of the fertile flowers. All these events can result in the serious reduction of fruit setting and fruit production, and consequently lead to alternate bearing. A shortage of water at this period, however, can be managed by watering olive trees. This is possible only in irrigated olive farms where water is available but not in rain-fed (non-irrigated) ones.

There are some other factors affecting fruit setting that are difficult or impossible for olive growers to control. High temperature and warm and dry winds can severely reduce fruit setting. This is because they cause dehydration, dryness and flower drop at the full bloom stage. However, the combination of high temperature, dry winds and a shortage of water can most severely damage flowers and fruit setting. The most favorable temperatures for fertilization and fruit setting in olive flowers range between 22 and 25°C. Temperatures below 15°C or above 30/32°C can cause serious problems in fruit setting. Therefore, temperatures above 32°C or even above 35°C that are forecast for the next 4-5 days in Greece may significantly reduce fruit setting and production of olive trees which are at the full bloom stage.

The degree of damage caused depends on the cultivar and other factors. For instance, a study of Greek cultivars in Crete showed that the pollen of ‘Mastoidis’ and ‘Kalamon’ cultivars was more resistant at 30°C than that of ‘Koroneiki’ and ‘Amygdalolia’ but at 40°C the pollen of all cultivars was similarly sensitive. Good nutritional and hydration status of olive trees positively affects the tree’s resistance to biotic and abiotic stresses such as high temperature. An extended period of high temperature increases the danger of damage, particularly at full bloom. Olive farms situated in south-facing sites are at higher risk of damage than those at north-facing sites since the latter usually have lower temperatures. The degree of flower damage is also dependent on the percentage of open flowers at the time of exposure to high temperature. Irrigated olive trees are at less danger of flower damage than those of non-irrigated olive farms.

Practical advice for olive farmers for reducing the risk of damage from high temperature at flowering

Irrigation of olive trees must be regular since it keeps the soil in good water condition that enables water transfer to leaves and flowers, thus reducing the danger of flower dehydration. It also helps in enhancing the air relative humidity that can modify and reduce the air temperature. However, farmers must take into consideration that overwatering olive trees in the flowering period will also remove nutrients from the soil by leaching (particularly nitrogen) which will negatively affect fruit setting and therefore must be avoided. The use of sprayers in this particular case (rather than the drop-irrigation system) has better results because it further increases the air relative humidity.

Optimum fertilization of olive trees, based on leaf and soil analysis, that keeps trees in a good nutritional status results in good growth and quality of flowers and a better resistance to stresses (such as the shortage of water or high temperature) and favor fruit setting.

Farmers must avoid any soil tillage, particularly in spring, which reduces soil water content and organic material. In contrast, they should perform all the practices that increase the soil water content and organic material in the soil, particularly in dry olive farms. Some of the agricultural techniques that should be applied in olive farms are the following: Mechanical mowing to control vegetation rather than soil tillage or using herbicides. Pruning and mowing residues should be applied to the ground surface or incorporated into the soil, rather than being burnt, in order to increase the organic content. Organic fertilizers such as farm-yard manure, the maintenance of a grass or crop cover on 30-50% of the soil area (between tree rows) or other agricultural techniques that are beneficial in increasing water content of the soil must be used.

Dr Stavros N. Vemmos

Emeritus Professor of Pomology, Agricultural University of Athens, Greece

President of 4Ε