As olive lovers, olive traders and, first of all, olive growers know, since a few years, European olive groves and European olive production are under serious threat. The reason being that olive trees, first in Italian Apulia, then in Balearic Islands, and thereafter in mainland Spain, France and other countries, have been attacked by a bacteria of America origin named Xylella fastidiosa.
This bacterium is transmitted by insects feeding at the plant’s xylem sap, specifically by the spittlebug. The bacterium colonises the xylem (plant’s vessels) and eventually blocks the water passage therethrough leading to the death of the plant or its significant parts.
Caption: Olive trees dying as a result of Xylella attack | Image credits © Shutterstock
The outbreak and ongoing spread of xylella is nothing less than catastrophic in terms of impact on economy, landscape, society and culture of the regions affected.
Teaming up for Integrated Pest Management to counter Xylella in Europe
18 months ago, olive growers and farmer associations together with research experts established a powerful consortium called BIOVEXO to study the threat and to develop solutions to combat the progressing disaster.
One of the first outcomes of their work has been a comprehensive scientific study on the status quo and an evaluation of the solutions available. Unfortunately, it’s been reaffirmed that non of the solutions works well and that the disease keeps spreading. BIOVEXO colleagues under the guidance of Dr. Stéphane Compant, AIT Austrian Institute of Technology, and Dr. Pasquale Saldarelli, CNR, National Research Council of Italy, outlined three basic mechanisms that are being developed and that have to be applied in parallel as a full-fledged integrated pest management (IPM) of Xylella:
1. Controlling the bacteria in the trees
Chemical and biological control
Since the first identification of Xylella fastidiosa in olive trees and the observation of the devastating damage caused in Apulian orchards by the associated disease, researchers produced a growing body of literature on the attempt top control the pathogen. The approaches involve mineral formulations, chemical compounds, natural products and microbial antagonists.
Caption: Electron miscroscopy of Xylella clogging water passages in olive trees | Image by courtesy of Dr. Angelo De Stradis, CNR
A series of studies has shown that alterations in mineral homeostasis of the plants mainly involving zinc, copper and calcium ions have significant effects on Xylella lifestyle (biofilm formation and growth rate). A longer-term evaluation revealed that bacteria concentration tended to decrease in trees regularly sprayed with a zinc-coper-citrid acid over 3-4 years.
Other strategies to control Xylella fastidiosa in infected olive trees included spraying with ammonium chloride or aluminium nanocrystals coated with chitosan; however, all these solutions haven’t lead to efficient Xylella disease control.
Caption: Treating olive trees | Image by courtesy of Enza Dongiovanni, CRSFA
Plant and microbial compounds
Research on these compounds is still the in-vitro stage. Generally, plant-derived phenolic compounds show mild inhibitory effects on Xylella growth. A new, very recent way examined is an approach attempting to modulate Xylella cell-to-cell-signalling with help of fatty acid molecules.
Impact of microbial ecology
It has been demonstrated that other microbial communities inhabiting Xylella-infected olive trees can act as potential biocontrol agents against Xylella in olive trees.
2. Controlling the insect transmitter
Vector control is the principal method available for controlling many insect-borne diseases. Xylella is being transmitted by the insect Philaenus spumarius, also known as spittlebug, and thus multiple studies have focused on its elimination or reduction (called in this context vector or transmitter).
Caption: Philaenus spumarius | Image by courtesy of Enza Dongiovanni, CRSFA
Control strategies target both the nymphal and adult populations, although targeting the immatures is more effective and sustainable. Nymphs have limited movement and do not contribute to the spread of the infection; chemical control of the adults needs to cover their whole life span from May to October and collides with the need to burden the plants themselves as little as possible.
Weed and ground vegetation removal
Various trials consisting of soil tillage were all able to significantly reduce the juveniles of the spittlebug species. More specifically, the use of systemic herbicide, pyroweeding and double tillage yielded the highest efficacy.
Field trials in Italy and Spain have shown that the most significant reduction of both nymph and adult populations has been achieved by substances neo-nicotinoids and, above all, pyrethroids. The formulations are typically sprayed on canopy. No formulations containing elements banned in the EU are being tested.
Appropriate timing for applying insecticides is a crucial aspect for the effectiveness of the control strategies: effort should be made in the early phase of the adult season, i.e. in the beginning of May. It has been noted that the spittlebug is not a very efficient transmitter compared to its cousin in North American pathosystems, but its population level and preference for olive plants compensate for the low efficiency.
Data on parasitoids are very preliminary but egg parasitoids have been identified both in Portugal and in Corsica.
Although very limited, attempts using kaolin or imidacloprid as potential insect repellents were realised in a new plantation in Italy. Use of these substances do not constitute a lasting prevention of infection of healthy plants however contributes to reducing and delaying the impact.
Pheromones and volatiles
In the framework of developing integrated pest management strategies, some studies investigated innovative approaches such a the manipulation of the feeding and sexual behaviour of the spittlebug. Four compound and specific vibrational signals have been identified to which the insect is responsive, which may allow for developing future control strategies: push-and-pull or attract-and-kill.
3. Plan breeding and grafting
Olive has a large (more than 900 cultivars) genetic and phenotypic variability in the Mediterranean area. Widely represented cultivars Ogliarola salentina and Cellina di Nardo are highly susceptible to Xylella whereas traits of resistance were find in cultivars Leccino and FS17®.
Caption: Grafting monumental olive trees with Xylella-resistant cultivars | Image by courtesy of Francesco Specchia
Following these findings, adoption of the grafting of Leccino on Ogliarola salentina and Celline di Nardo is under evaluation to save the centennial olive trees in Apulia. In addition, olive seedlings naturally grown in the epidemic area and surviving the infection are under screening to evaluate their suitability for further breeding.
Integrated Pest Management (IPM)
Since there are no approaches for the removal of the bacteria from the plant, the core of the IPM strategy to control Xylella relies on early detection and rapid application of phytosanitary measures, including, among others, of plant removal and transmitter control. A longer-term strategy consists then of the search for and adoption of resistant/tolerant species and cultivar. However, more effort is still needed to provide more complete understanding and truly comprehensive and sustainable solutions. This, together with the development of solutions in line with the European Green Deal ambition*, is the major aim of the BBI-JU-funded project BIOVEXO.
To read the full article, please go to https://www.mdpi.com/2076-2607/9/8/1771.
* An ambition to create a healthier and more sustainable EU food system, among others by reducing the use of chemical and more hazardous pesticides by 50% by 2030.