The quality of the soil and our various climates determine the quality and diversity of the olive oils produced.

I have an olive tree in my garden, that if I am honest, I don’t pay too much attention to. It’s big and shady, and probably very old, and apart from leaves falling, the only nuisance is the olives themselves – I am not partial to them, and every year I am almost knee-deep in them – most of which are wrinkly and sad looking by the time they hit the ground.


Olive Knot Disease

However, this year, something a bit sinister is going on with this tree. Some of the finer branches are dying off and they have no leaves or fruit, and there are some knobbly things up there that don’t look like they belong. Apparently, it’s olive knot disease - technically called Pseudomonas savastanoi. Its main characteristic is the formation of these lumpy things, called galls, and are bacterial, apparently spread by rainfall, and survive in gall tissue or epiphytically on twigs and leaves, spreading within the canopy by water splash or pruning tools. Olive tree varieties that undergo regular pruning are particularly vulnerable. Oh dear, this sounds like me, lopping off bits to keep the tree clear enough to walk beneath without being poked in the eye.

Olive trees are extremely resilient, and they've been known to tolerate frost, drought and even fire, re-growing and flourishing once again where few other plants could. You could say they are miracles of nature - but they are not resilient to everything.


Xylella fastidiosa (X, fastidiosa)

This is a devastating bacterium for Portugal, and the disease attacks and can kill century-old olive trees, severely diminishing yields. It spreads about 20km to 25km a year, and despite Europe being considered to be free of this disease, since 2013 the bacterium was identified as the cause of death of many olive trees in southern Italy. It’s native to the Americas, where it has caused disease in many crops, including citrus, coffee and grapevines.

Sometimes referred to as Pierce's Disease, this is a serious threat to grape production in Europe. It is spread by insect hosts that feed on the xylem sap of plants, transmitting the bacterium to other plants. This suggests a high potential for spread and the necessity for early monitoring. It isn’t fussy either - it has a wide range of host plants such as olives, citrus, grapevines and various ornamental plants, with the first three being important crops to Portugal.

In 2013, olive trees in the Apulia region of Southern Italy began exhibiting leaf scorch symptoms that were later confirmed to be caused by X. fastidiosa. Since then, thousands of olive trees have died, and X. fastidiosa has been detected in various plant species in France, Spain, and Portugal, and has been responsible for significant economic losses in the United States, Italy, and Brazil.


Xylella was identified in Portugal in 2019, but its symptoms had already been spotted in several European locations. All Mediterranean and olive-producing countries have created monitoring operations as requested by EU regulations. At present, when the disease is identified, the only available measures to stop it spreading are preventative -destroying affected trees and plants, and applying buffer zones around an outbreak site.

It has the potential to cause an annual production loss of 5.5 billion euros in the EU, affecting 70% of the EU production value of older olive trees (over 30 years old), and 35% value of younger ones; 11% of citrus; 13% of almond and between 1-2% of grape production in a scenario of full spread across the entire EU, according to the EU Commission for Food, Farming, Fisheries.

Control methods are being set up to try to control the spread of this disease, with quite specific infection zones and buffer zones being created, as at present, there is no known cure for diseased plants.

Amazing that something as small as a bacterium could cause so much damage.

Author

Marilyn writes regularly for The Portugal News, and has lived in the Algarve for some years. A dog-lover, she has lived in Ireland, UK, Bermuda and the Isle of Man. 

Marilyn Sheridan