Could Pistachio (<i>Pistacia vera</i>) Be a Suitable Alternative Crop for Olive-Growing Mediterranean Areas Affected by <i>Xylella fastidiosa</i> subsp. <i>pauca</i> ST53?

In the olive-growing areas of Apulia (southern Italy) where <i>Xylella fastidiosa</i> has caused enormous damage, there is a need to identify alternative crops. These could include pistachio (<i>Pistacia vera</i> L.), but it is critical to define the impact of the bacterium on this crop and what are the main phytosanitary threats for this species in the areas where the bacterium is now endemic. Therefore, we started evaluating infections caused by <i>X. fastidiosa</i>, the fungus <i>Neofusicoccum mediterraneum</i>, and other pathogens on four pistachio cultivars ('Kerman', 'Aegina', 'Lost Hills', and 'Napoletana') grown in areas where <i>X. fastidiosa</i> has been present for a long time. <i>X. fastidiosa</i> was detected only in one orchard (incidence: 18% 'Napoletana' and 55% 'Kerman') out of six surveyed orchards, with low bacterium concentration (1.67 to 5.98 × 10³ CFU ml^-1) and no symptoms. <i>N. mediterraneum</i> was retrieved in three orchards just on the cultivar Kerman but with high incidence (up to 30%) and infection level quantified as molecular severity (6.82 to 7.43); no other pathogens were detected. The <i>N. mediterraneum</i> representative isolates characterized in this study showed similarity with Spanish and Portuguese isolates. A confocal microscope analysis for this host-pathogen association suggested no differences in plant response to fungal aggression between the cultivars Kerman and Aegina, but just lack of latent inoculum in 'Aegina' plants, pointing to a possible nursery origin of the infection. Waiting for additional targeted experiments to clearly define host response of pistachio cultivars to <i>Xylella</i> spp., this study also points at <i>N. mediterraneum</i> as a potential threat to this tree crop new for the area.