Why do olive trees leaves turn yellow?

Olive trees are not only one of the most beautiful trees in the Mediterranean flora, but also one of the hardiest. However, we have all seen yellow in the leaves of an olive tree. The reasons for this can be many, some normal, others requiring remedial measures. 

Olive leaves usually live for two years on the tree, some maybe even three. These old leaves therefore turn yellow in the months of June and July in particular, just before they come off. This, of course, is part of the tree’s biological cycle. However, not all    yellowing is normal. Some causes of leaf discoloration are:

  • Lack of water

All trees, some a lot and some a little, need water to photosynthesize. In particular, the olive tree, although it is known to withstand drought conditions, needs a lot of water in the summer months, especially in our country, where temperatures are very high, but also in the early stages of its growth. Also, potted trees are known to require more watering, as the water in them dries out more quickly. Usually, yellow leaves due to lack of watering exhibit caramelization and wilting.

  • Excessive water

Trees don’t like overwatering, but only want it when their soil dries out. So all is fair enough! If too much water is dropped, the roots don’t get the air they need to breathe, so the tree doesn’t grow properly. It is important to stress that in case our soil is heavy (carries too much clay) the problem will be exacerbated. 

At this point, important to understand if the yellowing comes from the amount of water (too much or too little), we observe if the yellowing comes initially from the old leaves and not from the newer ones.

  • Weather conditions 

Weather conditions can cause premature aging of olive tree leaves. Intense heat, extreme drought or cold, frost and hail can affect the health of the leaves and cause them to turn yellow, indicating that they are undergoing stress, so-called hydrothermal stress. More susceptible are trees that have been recently transplanted, since they have not established a strong root system, and those that lack sufficient amounts of potassium, an element that helps make trees hardy.

  • Insufficient fertilization/nutrition

One of the main causes of leaf discoloration is food deficiencies, so we’ll discuss them a little more in depth so that we can better identify them. Olive leaves need nutrients, both macronutrients such as nitrogen (N), phosphorus (P) and potassium (K), and micronutrients such as boron (B), magnesium (Mg) and calcium (Ca) to keep them healthy and green. 

Nitrogen as one of the most important elements needed by the olive tree and in large quantities, especially in spring. If we observe yellowish-green leaves, small size, early leaf drop and few flowers per inflorescence, then it is very likely that we are talking about nitrogen deficiency. The recommended dose for an average aged, dry-grown olive (<400mm of irrigation) is 100gr/tree/100mm of water and 150gr/tree/100mm of water for areas above 700mm of irrigation. These amounts are indicative, as the soil and pruning of the tree have equal weight in the choice of the amount of fertilization. It is important to know that these amounts are added in early spring and autumn, but not in winter, as the tree is dormant and not producing sap at that time. Nitrogen is an element that, if not taken up directly by the trees, is lost to the soil and the atmosphere.

Yellowing of olive leaves when caused by elemental deficiency is usually due to nitrogen, potassium or boron. However, when discoloration is found in the periphery of the leaf starting from the top and at the same time several short shoots are seen, it is very likely a phosphorus deficiency.

In Potassium deficiency the leaf takes on a brass color (brownish-gold) starting from the top of the leaf blade. At the same time, the new leaves that emerge carry a very small size. Usually potassium deficiency is found in acidic and sandy soils, of course this is not the norm. It is recommended to use 100-150gr of potassium sulphate in a tree of average growth.

Boron in particular helps flowering and fruit setting. Its deficiency will even show if the young leaves show yellow tints at the edge. In fact, if the tree does not take up the Boron that it desires, the leaves will become distorted before they fall. The lack of Boron in the olive tree is often manifested in the form of a broom-like appearance of certain branches (many dry branches gathered together). 200-300 g of boron per tree is recommended if the tree is in full growth and every 3 years. Younger trees, need less fertilization. Always remember that it is more desirable to apply small amounts of fertilizer than larger amounts at the risk of burning your plants. 

As far as Calcium is concerned, yellowing of the leaves occurs at the edge (as in Borium), while at the same time the leaf veining is observed in old leaves to be light in color.

  • Diseases and parasites

Certain diseases and pests can also affect the health of olive leaves. 

By diseases we refer to fungi, bacteria and viruses. Below we will discuss the most common fungi that cause yellowing of olive leaves. 

The Cercospora cladosporioides fungus (Cercospora cladosporioides), is well known to olive lovers. It is favoured by humidity and is carried by rain and wind. Susceptible varieties are Koroneiki, Thrubolia, Amfissis, Tsunati and Kalamon. This fungus is easily recognised by the infestation it causes on old vegetation, with the characteristic chlorotic spots, which slowly turn yellow, on the upper surface of the leaves. At the same time, a grey discolouration appears on the lower surface, these are the fungal conidia. Spraying with copper-containing preparations is recommended.

 Another case of a fungus that causes yellowing of leaves is cycloconium. Cycloconium is caused by the fungus Spilocaea oleagina, and is very quickly identified by the characteristic grey circular marks. The susceptible species here are Lianolia, Chondrolia and Amfissis, while Koroneiki and Megaron are resistant. Of course, in this case too, spraying with copper-containing preparations is recommended.

As far as insect pests of the olive tree are concerned, most of them are active in spring and summer. Some of them are the mealybug, the red spider mite and the cockroaches. To combat them, we spray with insecticides that are always approved. However, there is a more ecological alternative, which in small infestations is the most suitable. Specifically, in 1L of water, add 1 tablespoon of ground green soap and 1 teaspoon of pure alcohol and spray. 

In conclusion, to deal with the problem of yellowing leaves on your olive tree, it is important to provide sufficient water when the soil is dry, feed it properly and monitor the health of its leaves for early detection of possible diseases or pests.