Sour Orange Microbiome Is Affected by Infections of Plenodomus tracheiphilus Causal Agent of Citrus Mal Secco Disease

Mal secco is a severe vascular citrus disease (MSD) caused by the mitosporic fungus Plenodomus tracheiphilus (Pt). The pathogen enters through wounds on the above- and below-ground parts of the tree. The susceptible species sour orange (Citrus aurantium) is the most commonly used rootstock for lemon trees in Italy. In this study, sour orange seedlings were wound-inoculated with P. tracheiphilus in leaves or roots. Six months post-inoculation, the microbial communities of rhizosphere, endorhizosphere, and xylem endosphere samples from inoculated and healthy plants were analyzed by 16S rRNA and ITS (internal transcribed spacer) amplicon sequencing. The DNA of Pt was quantified by real-time PCR in the three compartments. According to our results, the endorhizosphere of root-inoculated plants showed the highest concentration of the pathogen DNA. Bacterial populations of potentially beneficial taxa (e.g., Pseudomonas and Burkholderia) were depleted in the rhizosphere of the inoculated plants. Infection through leaves and roots also produced a network-wide rewiring of microbial associations in sour orange roots. Overall, our findings revealed community-level changes induced by Pt infection in the sour orange root and xylem microbiome, providing further insights into the beneficial multispecies interactions in Citrus-associated microbial communities.