Could Xylella destroy the European almond industry?
– It is complex, but almonds are threatened by Xylella
As Marilyn Munroe sang ‘Diamonds are a Girl’s Best Friend‘, could we sing ‘Almonds are a (spittle)Bug’s Best Friend’? Well, maybe that’s a little bit of a stretch; adult spittlebugs actually prefer to feed on olive trees, but as we have learned in previous blogs, Xylella can cause disease in a large number of plant species. Several Prunus species, such as almond, peach, and plum, are heavily stressed by Xylella. The BIOVEXO project is concerned with Xylella in two plant species in particular: olives and almonds. We have previously looked at what Xylella has been doing to olives trees. In this blog we’re going to explain a little more about almonds: why they are good for us, how Xylella can affect them, and what the BIOVEXO project is doing to protect them.
Almonds: hard, healthy, historical.
Almonds are part of the Prunus genus, along with apricots, cherries, peaches, plums, and nectarines. Whilst considered a nut for culinary purposes, the fruit of the almond is, in fact, a drupe… But, what is a drupe? It’s a stoney fruit (a soft outer layer, containing a hard inner layer, which protects the seed (the almond)).
Almonds (Prunus dulcis L.) are indigenous to the Central Asian and Eastern Mediterranean regions and, by the tenth century AD, had been introduced to the entire Mediterranean basin. Almonds grow best in climates with warm, dry summers and mild, wet winters. This means they grow in exactly the same kind of climate that is also favoured by the spittlebug.
Through the Spanish colonization, Almonds were also introduced to the Americas, where they have become an important crop, with the USA now by far the largest almond producer globally.
Eating almonds can have significant health benefits. Almonds can help lower cholesterol, lower blood pressure, improve gastrointestinal health, and provide valuable nutrients, like magnesium and vitamin E.
And, as well as being healthy, they’re also yummy! Almonds can also be enjoyed in many cakes and sweets, either whole, diced, ground, or in the form of marzipan.
Maybe the combination of health benefits and tastiness is why almonds have been mentioned in everything from ancient Greek medical texts to the bible.
In ancient Rome, newlyweds were showered with almonds, which were then believed to be a fertility charm. They’re even big in art – Vincent van Gogh famously painted almond blossoms in Arles and Saint-Rémy, southern France.
Hopefully, all of that is enough to make anybody an almond fan! So, let’s carry on getting to know them.
The Almond Lifecycle.
Springtime in Spain, for example on the Balearic island of Mallorca, sees a flood of beautiful, delicate light-pink blooms (just like those ones Van Gogh painted). Between mid-February and mid-March, almond tree buds burst into beautiful white and light-pink blooms. When the trees blossom, they attract honey bees in search of pollen and nectar. This means these blossoms are the trigger for beekeepers to bring hives into the almond orchards to help pollinate the flowers.
As the bees move around from tree to tree, they help to pollinate the almond trees. Without the bees we would not have almonds: every almond exists thanks to the work of a bee! Over the course of the next few months, the blossoms drop and almonds mature, growing a fruit with a soft outer shell, with a fuzzy skin, and a tough, inner shell to protect the seed/kernel (the almond) inside.
By early- to mid-summer, as temperatures rise and rainfall decreases, the fruits begin to split open, exposing the inner shell and allowing it to dry and harden. As we learned in our last blog, this is also the time of year when spittlebugs, the carriers of Xylella, are heading to the trees to feed. As they continue to mature, the fruits turn from green to golden-yellow, to light brown.
Between late summer and late autumn, the almonds are ready to be harvested. Around this time, the almond trees begin producing the buds that will turn into the flowers for next year’s crop. This means farmers have to be particularly careful to ensure the trees are healthy.
Over the winter, almond trees lose their leaves and enter a ‘rest’ period. This enables them to store up energy and nutrients for the spring and the next period of growth, when the buds start swelling and become bright blossoms again. The cycle has been completed.
A similar cycle plays out each year in almond orchards around the world. But sometimes this happy, bucolic pattern gets broken…
Maybe the first sign that all is not well is the appearance of dried out edges on leaves. Has it just been a dry spell? Do the fields need more water? Or is there something choking the tree from the inside? Is death silently stalking the orchard? These are some of the questions that have been increasingly concerning almond farmers in Mallorca and other regions around the Mediterranean.
A silent killer in our midst?
Xylella has been found in Almonds in many locations, ranging from Argentina and California, to Iran and Israel. Evidence suggests Xylella was actually present in almonds in Europe from around 1993 – 20 years before it was identified in olives in Apulia, Italy. In almond trees, the Xylella bacterium caused Almond Leaf Scorch Disease (ALSD).
The first known observation of what is now called ALSD dates to 1930 in California. In Europe, ASLD caused by X. fastidiosa subsp. Fastidiosa was first detected in Mallorca in 2017. ALSD caused by X. fastidiosa subsp. Multiplex was also detected in Alicante in 2017. These subspecies act differently on almond trees, with the former causing more severe symptoms. Additionally, different almond cultivars react differently to Xylella. Two different subspecies of Xylella can also exist simultaneously and cause ALSD in the same orchard, or even within the same plant.
In some trees, only a few branches display symptoms whereas, in others, the entire scaffold in both the lower and upper canopy may suffer.
Symptoms of ALSD consist of scorching of leaves, often with a golden yellow band between the inner green parts and the brown dead tissue around the leaf edges. Necrosis generally starts at the leaf margins and progresses to the midrib. However, before symptoms even begin to become visible, farmers often notice a reduction in productivity, with trees producing fewer almonds. Once symptoms do become visible, they progress through the growing season. Severely affected trees show a golden yellow colour in the entire canopy. This is commonly known as ‘golden death’.
As was the case with olive trees in Apulia in Italy, the devastation of almond trees, an important icon of the agricultural landscape on the island of Mallorca, has had far-reaching social consequences. ALSD is estimated to affect 79% of almond trees in Mallorca. Evidence of it has been identified via qPCR testing on saw dust samples taken from tree rings, dating infections back to 1998 and it has been estimated that Xylella could, in fact, have reached Europe as early as 1993.
This first transmission was likely caused by infected plant material imported from California being grafted onto European root stock. As was the case with olive trees in Apulia, Italy, spittlebugs have been proven to be the vector of ALSD.
Symptoms of ASLD can be triggered in infected trees by water stress (e.g. extended periods of high temperature (Tmax > 33 °C) and low humidity (RH < 60%)). Accordingly, symptoms may manifest more slowly in cooler, more humid climates. This does not bode well in the face of climate change.
What is being done to protect almond trees?
Due to the threat it faces, Europe has become a centre of research on X. fastidiosa and almond trees. Between 2013 and 2020, approximately a third of the total number of studies published on Xylella in almonds were related to European research. But, this research has focused mostly on the spread of the ALSD, the strains of Xylella causing infections, and vectors involved in transmitting the bacteria.
Over seven-thousand plant samples, covering 274 plant species, were analysed for Xylella fastidiosa at the Balearic Islands Official Plant Health Laboratory (LOSVIB) between 2016 and 2019. As a result, three Xylella fastidiosa subspecies were identified. However, the focus on the bacteria means that, unlike studies undertaken in North America, no resistant or tolerant almond cultivars have been identified in Europe.
In California, the almond cultivars “Butte” and “Carmel” are considered Xylella-resistant due to their ability to reduce bacterial load in the winter months. This means they usually show few symptoms and can survive even in fields severely attacked by ALSD. The disease has also been reported to be less common in “Mission,” “Aldrich,” and “Padre” cultivars. Consequently, some researchers are beginning to look at ways of grafting resistant cultivars onto existing Almond trees in Europe.
Unfortunately, simply removing infected branches does not cure an almond tree once it has been infected. As there are no chemical or nutritional treatments that can control or cure ALSD, if the orchard is young (5–10 years old), the best course of action may be to remove all the infected trees in their entirety. In older orchards (16 –20 years old), it may be more cost-effective to keep infected trees, because the entire orchard is normally replaced when trees reach between 22 and 25 years of age and infections will probably not significantly impact yields before then. It appears that the latency period for ALSD may change depending on the time of year that infection occurs; almond trees infected in the spring seem to more quickly develop more severe symptoms when compared to trees infected in the summer and autumn.
Xylella is resilient once it becomes established in a region: in Iran, it’s been reported that Xylella survived overwinter in root tissues in almond orchards with winter temperatures even below -15°C. The BeXyl project is, amongst other things, investigating whether thermal treatment can enable safe plant material exchange.
Final words:
We’ve now had a chance to learn about the important role almonds can play in our diets, the important role they do play in many local and regional economies in different parts of Europe (and the world), and the threat Xylella poses to their continued cultivation in Europe. If you’d like to follow the work of the BIOVEXO project and stay up to date on the fight against Xylella, please sign up to our Newsletter and follow us on Twitter, Instagram, LinkedIn, or Facebook.
