This research project is co-ordinated by the Instituut voor Landbouw- en Visserijonderzoek (ILVO), and includes
the Université catholique de Louvain (UCL) as well as ULB. It targets some key aspects in the Xylella fastidiosa (Xf)
epidemiology that could have an important impact on plant health in Belgium, i.e.:
1. the disease susceptibility of a set of plant species that could be at risk in Belgium and their suitability as sentinel plants for use here as well as in regions with disease outbreaks; 2. environmental factors such as temperature and plant fertilization as possible inducers of disease. 3. the presence of possible insect vectors in Belgium, their life cycle and their capacity to transmit the bacterium.
Xf is not an homogenous bacterial species. The taxon Xf consists of several variants which are resolved at the subspecies level. These Xf variants differ in vector transmission, disease severity, host range and response to environmental factors. It is therefore imperative to elaborate the investigations with a strain from the relevant taxa, i.e X. fastidiosa subsp. multiplex, X. fastidiosa subsp. fastidiosa, and the strain isolated from olive in the outbreak in the Puglia region, which is considered as a variant of X. fastidiosa subsp. pauca.
Xf is globally associated with at least 300 plant species belonging to 61 families. The broad range of possible insect vectors and host plants implies that the potential of the bacterium to establish and spread in Belgium should be considered as high. Through the decennia of the uncontrolled imports of host plants from the American continent, there is even speculation that Xf may be present in the EU, but has remained unnoticed. In fact, the presence of Xf in a plant is not a priori correlated with disease, most infections stay asymptomatic. A high number of potential plant hosts, cultivated or not, are present on the Belgian territory. However, their response to the Xf variants have never been studied up till now. Investigation of many plant species is not feasible within the scope of this project. Therefore, pertinent plant species are selected which figure on the extended host lists for Xf; Vitis vinifera and Prunus domestica, which are reported as important hosts and represent a growing market in Belgium; Quercus petraea, widely present in Belgian forest and tree nurseries; Salix alba, widely present in the agricultural landscape, but also studied at ILVO as a host of the xylem-limited bacterial pathogen, Brenneria salicis; and finally the herbaceous plant Catharantus roseus which is frequently used as experimental host because of its permissiveness to infections of Xf. In controlled inoculations in contained facilities we will monitor Xf in these plants and evaluate the possible use of these plant species for surveys on Xf occurrence and/or symptoms in present and different/changing environmental conditions. Moreover, the status of these plant species as pathogen reservoir (symptomless) or as a real host associated with development of the disease is important epidemiological information.
Mostly Xf stays silent in the plants, but in some specific it will develop into disease. Factors that have a decisive influence on this transition from silent (symptomless) to disease are not understood, and this greatly hampers a well-founded control policy. We want to generate new information on this crucial event in the Xf epidemiology. Development of Xf disease is possibly tempered by non-conducive environmental conditions coexisting up till now in the northern European countries. We will investigate the influence of plant fertilization and temperature as possible triggers for transition to the pathogenic stage. In the symptomless stage, Xf is present in low concentrations in the xylem network, whereas for the pathogenic stage Xf needs to reach high concentrations inside the xylem. Xf pathogenesis is quorum dependent. In earlier research at ILVO it was demonstrated that nitrogen fertilization can drive the population of an endophytic bacterium to high concentrations in the xylem of willow, which was associated with the onset of disease by this bacterium. Fertilization of the plant causes changes in the composition of the xylem sap becoming rich in nutrients for bacterial growth. We will study if fertilization of the plant has a similar effect on Xf growth and pathogenesis. Of an even importance can be the factor ‘temperature’. Outside temperatures during the year have an important influence on the concentrations of wood endophytes. In fact, endophytic populations are yearly down-regulated by lower winter temperatures. This fact possibly restricts Xf disease outbreak when combined with a normal winter regime in BE, but poses a risk when going to warmer winters. It has been shown that optimal Xf growth occurs between 25 and 32°C, whereas temperatures below 12 to 17°C reduce Xf survival and proliferation in the plant. Indeed, severe Xf disease is especially reported in warmer climate regions. Our approach is to test the growth of the Xf variants in xylem sap from selected host plants under different temperatures, and this in combination with the fertilization parameter.
The pathogen is a strict xylem-inhabitant, being vectored by xylem-sap feeder insects. Efficient insect vectors represent a risk of epidemic spread of Xf. All xylem-fluid feeding insects are considered as potential vector. However, there is a general lack of data on the transmission capacity of reported insects vectors and transmission efficiency varies substantially depending on insect species, host plant and Xf genotype (Redak et al., 2004; Lopes et al., 2010). Beside this fact, most of the xylem-feeding insects reported in Belgium and in Europe have never been studied for their Xf transmission capacity. The species identified as a vector in the present outbreak in Puglia (Italy), Philaenus spumarius (Hemiptera, Aphrophoridae), is widely present in Belgium, as well as four other species and very likely to be vectors as well, Aphrophora alni, Aphrophora salicina, Cercopis vulnerata and Cicadella viridis.