Investigations on the transmission of the bacterium Xylella fastidiosa by insect vectors
Author(s)
Ben Moussa, Issam Eddine
Date Issued
March 24, 2017
Type
Doctoral Thesis
Abstract
The quick dissemination of Xylella fastidiosa in olive trees in the province of Lecce (southern Italy) prompted an investigation to identify the local insects which are likely to be involved in the transmission of the bacterium, to study their seasonal dynamics and to evaluate their role in the disease epidemiology. The study focused on the Auchenorrhyncha species (sharpshooter leafhoppers and froghoppers or spittlebugs), a group that includes known vectors of X. fastidiosa. Adults of these species were monthly sampled over a two years period from four severely infected Apulian olive groves. Fifteen Auchenorrhyncha species were captured and identified in that area, in three of them, i.e. Philaenus spumarius, Neophilaenus campestris and Euscelis lineolatus, X. fastidiosa was detected by PCR. This study revealed that adults of P. spumarius, already known vector of the bacterium, are greatly dominant compared to the other species. Most of the captured species were abundant in summer and their adults appear in March-April, except for E. lineolatus that overwinters as adult. Transstadial or transovarial transmission was excluded since no Xf-positive insect sample was never found in adults of the new generations of the three species. According to these findings and considering the lack of latent period for X. fastidiosa transmission, preventive control measures against juvenile forms of the vector P. spumarius should be recommended from late March to mid-April until demonstrating the transmission ability of the other two species. From another hand, an evaluation of the possibility to utilize the Xf-positive insects found in the buffer zones (so called “Spy Insects”) as indicators of the presence of the bacterium in apparently uncontaminated areas, was carried out. From May to June 2014, insects were collected from a designed buffer zone in a punctiform Xf-infected area. The bacterium was successfully detected by PCR in the same 3 out of 6 species of Auchenorrhyncha captured. Infected adults of these species were detected throughout the buffer zone at a distance of 1 Km far from the infection site. Therefore, adults of these three species have an important role as “Spy insects” to early reveal the presence of the pathogen in apparently
free areas before symptoms become visible on the plants. Finally, X. fastidiosa contain endoglucanases in its genome. These enzymes cleave the β-1,4 glycosidic linkages composing the exopolysaccharides matrix protecting bacterial biofilms. Once acquired, X. fastidiosa multiply and colonizes the insect foregut by forming biofilms. We speculated that the endoglucanases may contribute in biofilm dissolution and interrupt vector transmission of the bacterium as a biological control measure. For this, we tested the acquisition, retention and transmission of a X. fastidiosa endoglucanases minus mutant by an efficient vector. In order to explore this effect, we conducted a laboratory-based transmission assays with artificial feeding sachets and results were obtained through quantitative PCR and scanning electron microscopy. It was shown that endoglucanases modulate bacterial adhesion to the insect cuticule over time and consequently affect the retention period of X. fastidiosa.
Additional information
Dottorato di ricerca in Scienze delle produzioni vegetali e animali
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