Climate tolerances of Philaenus spumarius should be considered in risk assessment of disease outbreaks related to Xylella fastidiosa

The bacterium Xylella fastidiosa (Xf) is an invasive insect-borne pathogen, which causes lethal diseases to important crops including olives, citrus, almonds and grapes as well as numerous forest, ornamental, and uncultivated plants. Outbreaks of Xf-related plant diseases are currently occurring in the Mediterranean region, causing substantial losses to various agricultural sectors. Several models have recently been published to identify which regions are at highest risk in Europe; however, such models did not consider the insect vectors, which constitute the key driver of short-range Xf spread. We fitted bioclimatic species distribution models to depict the macroclimatic preferences of the meadow spittlebug Philaenus spumarius L. (1978) (Hemiptera: Aphrophoridae), the major epidemiologically relevant vector currently responsible for Xf spread in the Europe. Many regions of Western Europe and Mediterranean basin are predicted by models as highly climatically suitable for this vector, including all regions where severe Xf have occurred so far. Conversely, the driest and warmest areas of the Mediterranean basin are predicted as little suitable for P. spumarius. Models forecast that agricultural-important parts of the southern Mediterranean area might experience a substantial decrease in climatic suitability for P. spumarius by the period 2040–2060. Areas predicted as highly suitable just for the bacterium but not optimal for this vector are apparently still free of severe Xf outbreaks, suggesting that climate tolerances of P. spumarius might partly explain the current spatial pattern of Xf outbreaks in Europe and should always be considered in further risk assessments.

     

Evidence of Migration and Endophytic Presence of Agrobacterium tumefaciens in Rose Plants

Agrobacterium tumefaciens was isolated from stem tumors of several rose cultivars showing that the bacterium is the causal agent of aerial galls in rose plants. No differences were observed in the characteristics of the Agrobacterium isolates from crown or aerial galls. Stem inoculation of ten rose cultivars showed that all of them were susceptible to A. tumefaciens but differences in the size of the resulting tumors were observed. The movement of A. tumefaciens in rose plants was demonstrated using two wild type strains and two antibiotic resistant mutants. Three months after inoculation, the inoculated strains were recovered in the roots, crown and below and above the inoculation site but low numbers of pathogenic Agrobacterium cells were isolated. New tumors appeared in 5% of the noninoculated wounds. A. tumefaciens was isolated from the stem at different distances from the tumor in naturally infected plants. In symptomless commercial plants, the isolation from the roots, crown and at different stem levels demonstrated the existence of systemic and latent infections in rose. Direct isolation using a nonselective and selective media with or without a previous enrichment step were efficient methods for isolating tumorigenic Agrobacterium from the different parts of rose plants.     

Integrated approach for detection of nonculturable cells of Ralstonia solanacearum in asymptomatic Pelargonium spp. cuttings

Ralstonia solanacearum (biovar 2, race 3) is a soil and water-borne pathogen that causes serious diseases in several solanaceous hosts. It can also infect geranium plants, posing an important threat to their culture when latently infected cuttings are imported from countries where the pathogen is endemic. R. solanacearum can be present in very low numbers in asymptomatic geranium cuttings, and/or in a particular stressed physiological state that escapes direct isolation on the solid media usually employed. Consequently, an integrated protocol has been developed to analyze asymptomatic geranium cuttings routinely. The first screening tests include isolation and co-operational-polymerase chain reaction (Co-PCR), based on the simultaneous and co-operational action of three primers from 16S rRNA of R. solanacearum. This method was selected as the most sensitive one, able to detect only 1 cell/ml including nonculturable cells. When isolation is negative but Co-PCR is positive, the bioassay in tomato plants is proposed, since stressed bacterial cells or those present in low numbers that do not grow on solid media can be recovered from inoculated tomato plants and retain pathogenicity. This methodology has been demonstrated to be useful and has allowed us to assess the relevance of the physiological status of bacterial cells and its implications in detection. It also reveals the risk of introducing R. solanacearum through asymptomatic geranium material when relying only on bacterial isolation.