en
Slovenščina
Italiano
Over the past few years, olive growers have found themselves asking the same questions every summer. Why is the fruit dropping? Is it the heat? Too little rain? Brown marmorated stink bugs? Fungal diseases?
In reality, none of these factors is usually the sole cause. More often, they act as a tipping point. Whether a tree can withstand the stress or begins to shed fruit depends to a large extent on its physiological condition, which develops over the preceding months.
A tree in good physiological condition has sufficient leaf area, a well-developed root system, and adequate reserves of water, nutrients and carbohydrates. Such a tree can continue supplying developing fruit even during a short drought or heatwave. The situation is very different for a tree that was overloaded with fruit the previous year or suffered from drought, nutrient deficiencies, disease or other stress factors. It enters the new growing season with lower energy reserves and is therefore much less able to cope with additional stress.
This is why the crop does not begin in spring, but already the year before. Inflorescence development starts around two months before flowering, while the floral organs complete their development two to three weeks before flowering. For this process, the tree relies almost entirely on reserves accumulated during the previous growing season.
Olive trees produce an exceptionally large number of flowers – a mature tree may develop around 500,000. This is no coincidence, but an evolutionary adaptation that increases the chances of successful pollination and fertilisation. Even so, only about one to two per cent of flowers ultimately develop into fruit by harvest time. Most are shed during natural physiological thinning, through which the tree adjusts the number of fruits to its available resources.
Figure 1 – Flower structure
A tree may appear perfectly healthy from the outside, while its physiological condition has already been weakened by unbalanced fertilisation, reduced leaf area, for example due to olive peacock spot, wood diseases, or damage to developing fruit caused by harmful organisms such as the brown marmorated stink bug. As long as conditions remain favourable, these limitations often go unnoticed. But when heat and drought occur during the most sensitive stage of fruit development, water shortage causes the stomata to close, photosynthesis declines, and less sugar is produced to support fruit growth.
When the available resources are no longer sufficient for all the developing fruit, the plant triggers natural thinning through hormonal signals. During this process, fruit resulting from unsuccessful fertilisation or containing damaged or poorly developed embryos is more likely to be shed, while the remaining resources are directed towards the development of healthy fruit.
However, not all fruits have the same prospects from the very beginning. At first glance, olive flowers appear almost identical, yet there is an important difference between them. Some are perfect, or bisexual, flowers. These have developed stamens and a normal ovary containing an ovule, allowing a fruit to develop after successful fertilisation. Others are functionally male flowers, in which the ovary is rudimentary or aborted. These flowers release pollen, but can never develop into fruit.
Their proportion is primarily a varietal characteristic. Some cultivars, such as ‘Oblica’, produce significantly more functionally male flowers than others, although the tree’s physiological condition and the weather during flower-bud development also have an important influence.
Figure 2 – Cross-section of a young fruit
Because a low proportion of perfect flowers could in itself explain a lower yield, this was the first factor we examined at the Institute for Oliveculture at ZRS Koper as part of the public olive-growing service. Four years of monitoring the ‘Istrska belica’ cultivar at three representative locations in Slovenian Istria showed a very high proportion of perfect flowers, ranging from 75 to 93 per cent.
This indicated that, for this cultivar, the reason for the subsequent reduction in yield was unlikely to lie in flower quality. Attention therefore shifted to the next stage of development – the period after fertilisation.
Following successful fertilisation, the ovule develops into a seed, within which the embryo forms. The seed is surrounded by the developing stone. In a healthy fruit, the embryo develops normally. In others, its development may stop due to abiotic factors such as heat, drought or nutrient deficiencies, or biotic factors such as diseases and pests, even though the fruit continues to grow externally.
The stone and flesh may continue developing normally, meaning that without cutting the fruit open or carrying out an anatomical analysis, such fruit generally cannot be distinguished from healthy fruit.
For this reason, we focused our research on embryo development. We wanted to determine when damage begins to occur, how it is related to weather conditions, and whether its incidence can be reduced through timely agronomic measures.
An analysis of meteorological data for Slovenian Istria covering the period 1961–2026 revealed a clear pattern. While the timing of flowering and fertilisation has remained relatively similar over the decades, the greatest changes have occurred between 20 and 60 days after flowering – precisely during the period of most intensive embryo development.
In recent decades, the number of days with maximum temperatures above 35°C has increased markedly, while both the number of rainy days and total precipitation have declined. Long dry spells without significant rainfall are no longer exceptional; they are becoming increasingly common precisely during the olive tree’s most sensitive stage of development.
Our results indicate that heat and drought are often not the sole cause of reduced yields, but can intensify the effects of factors that have already weakened the tree. Fruit development is influenced by the tree’s nutritional status and overall health, as well as by diseases, pests and the plant’s water status. When the demands of developing fruit exceed the tree’s capacity to support them, disruptions in embryo development and more pronounced natural fruit thinning may occur.
We cannot control the weather, but we can significantly influence the tree’s physiological condition. This is not built up over a matter of days, but throughout the entire year: through balanced pruning that maintains an appropriate ratio between leaf area and fruit load, adequate nutrition, fertile soils with sufficient organic matter, the considered use of biostimulants, effective protection against diseases and pests, and timely irrigation. Only a tree in good physiological condition can successfully complete the development of a larger proportion of its fruit during periods of heat and drought.
Long-term adaptation of olive growing to increasingly hot and dry conditions therefore requires adjustments to the entire production system, rather than simply responding to individual heatwaves.
The most effective protection against the effects of climate change is therefore not simply irrigation during the driest periods, but ensuring that the tree enters the embryo-development stage in excellent physiological condition. Today, the first sixty days after flowering are becoming increasingly decisive in determining how many flowers will ultimately develop into high-quality fruit by autumn. The most important agronomic measures are therefore not taken when a heatwave arrives, but several months earlier, when we create the conditions that allow the tree to enter the embryo-development stage in the best possible physiological condition.
Authors: Dr Maja Podgornik, Michelle Umer, Rok Babič, Jakob Fantinič
Acknowledgements
The authors sincerely thank the Public Service for Olive Growing for funding the monitoring of olive phenological development and the research activities. Special thanks go to Filip Sekuloski for his assistance with the processing of meteorological data, and to all the growers who made it possible to carry out measurements and sampling in their olive groves.