The olive quick decline syndrome (OQDS) is a disease that appeared suddenly a few years ago in the province of Lecce, Salento peninsula (south-eastern Italy). The major incitant of the disease is Xylella fastidiosa, a quarantine pathogen of American origin whose unwelcome introduction in the area has created much disturbance because: (i) the dramatic damage suffered by the olive groves where the pathogen has established itself; (ii) the alarm that this finding has raised in a country (Italy) whose olive/oil industry is a primary asset, and in the European Union, which is facing the first confirmed record in its territory of this alien and much feared microorganism.
OQDS is characterized by the presence of leaf scorching and desiccation of twigs and small branches, that prevail first in the upper part of the canopy (Fig. 1A), then extend to the rest of the crown, which acquires a burned look (Fig. 1B). The more seriously affected plants are heavily pruned by the growers to favour new growth which, however, is scanty and dessicates in a short while. The skeletal-looking trees push a multitude of suckers from the base (Fig. 1C) and survive for some time, i.e. as long as the roots are viable.
As to disease incidence, a rough estimate indicates that the whole area where multiple and disperse OQDS foci occur amounts to about 230,000 hectares. However, the infected surface area given over strictly to olive (i.e. the totality of the symptomatic olive groves taken together) may not exceed 9,000 hectares, which accounts for about one million plants. These are huge numbers that are liable to increase over time, considering that the infection foci, which now have a scattered distribution, will tend to coalesce.
X. fastidiosa is a bacterium with an unusual epidemiology (plant-to-plant transmission occurs only via insect vectors), that infectes a wide range of hosts (309 plant species belonging to 193 genera, according to a recent list issued by the European Food Safety Authority). It invades, multiplies and occludes the plant's xylem vessels, thus impairing water uptake. In the case of olive, the damage may be aggravated by the presence of fungi of different genera, Phaeoacremonium and Phaemoniella in particular, but also Pleumostomophora and Neofusicoccum, which colonize and necrotize the sapwood. Besides olive, the Salentian strain of X. fastidiosa infects in nature a number of woody (almond, cherry) and shrubby (oleander, broom, rosemary, Acacia saligna, Polygala myrtifolia, Westringia fruticosa, Rhamnus elaternus, Myrtus communis) hosts but not grapevines and citrus, as based on the absence of overt symptoms and the negative results of laboratory analyses of 380 individual vines and 350 citrus trees growing within, or next to heavily infected olive orchards.
A clue to the search for X. fastidiosa in OQDS-affected olives was given by: (i) the symptoms, which recalled very much the severe leaf scorching of fruit and shade trees induced by this bacterium, as described in the north American literature; (ii) the modality of disease spreading, which was compatible with that of X. fastidiosa infections.
The Salentinian strain of X. fastidiosa is readily and dependably detectable by serological (ELISA) and molecular (PCR) methods. It was isolated in culture from different hosts and identified as a genotype of X. fastidiosa subsp. pauca, genetically identical to an isolate of the same subspecies from Costa Rica, from where it may have been introduced into Apulia. The analysis of the complete genome sequence (a DNA molecule 2,507,614 bp in size) of the olive strain confirmed this identification.
Philaenus spumarius (meadow spittlebug), a froghopper quite common in the Salento area where it thrives on olive, was experimentally identified as the main vector. In fact, a very high percentage (in excess of 70-80%) of the spittelbugs captured on infected olives in summer 2014 were shown to carry the bacterium and were able to transmit it to an experimental (periwinckle) and the natural (olive seedlings) host.
Current knowledge tells that disease eradication and sanitation of Xylella-infected plants, olive included, are unfeasible. Thus, strategies are being enacted for restraining the spread of the pathogen and vector(s) within the boundaries of the currently infected zone by implementing a "containment plan" designed by a recently appointed government Commissioner. Cases of OQDS used to be confined to the province of Lecce until a short while ago, when a small focus has popped up in the neighbouring province of Brindisi. The new outbreak has forced to move north the zone which had been selected for a sort of "last stand" against the spreading of OQDS. The measures envisaged to this aim are mechanical weeding in spring, to kill as many juveniles as possible of the vector, which thrive especially on weeds, followed by insecticide treatments to olive trees, on which the adults move after moult and live happily. These measures are accompanied by the more drastic and unpopular one: uprooting the infected olive trees (many of them are several century-old giants) and the surrounding ones, in accordance with the requests of the EU. The hope is that this strategy will succeed if the envisaged measures are implemented quickly and thoroughly. Dealing with OQDS within the widely infected zone is another business, which requires targeted measures and much experimental work to be done.
Ph: Olive quick decline symptoms in the initial (A), advanced (B) and final (C) stages. The trees in C are pushing suckers from the base which, in the course of time, will decline and die with the tree.
Giovanni Martelli – Università di Bari - firstname.lastname@example.org