The maintenance of an intact water column in the xylem lumen several meters above the ground is essential for woody plant viability. In fact, abiotic and biotic factors can lead to the formation of emboli in the xylem, interrupting sap flow and causing consequences on the health status of the plant. Anyway, the tendency of plants to develop emboli depends on the intrinsic features of the xylem, while the cyto-histological structure of the xylem plays a role in resistance to vascular pathogens, as in the case of the pathogenic bacterium Xylella fastidiosa. Analysis of the scientific literature suggests that on grapevine and olive, some xylem features can determine plant tolerance to vascular pathogens. However, the same trend was not reported in citrus, indicating that X. fastidiosa interactions with host plants differ by species. Unfortunately, studies in this area are still limited, with few explaining inter-cultivar insights. Thus, in a global context seriously threatened by X. fastidiosa, a deeper understanding of the relationship between the physical and mechanical characteristics of the xylem and resistance to stresses can be useful for selecting cultivars that may be more resistant to environmental changes, such as drought and vascular pathogens, as a way to preserve agricultural productions and ecosystems.

Acute oak decline (AOD) is a multifactorial disease that affects European oaks and represents a growing threat to forests. The disease results from a complex interaction between biotic and abiotic factors: the various environmental stresses, which vary depending on the area in question, and generally increased by climate change, predispose trees to attack by opportunistic pathogens. Among them, we focused on a bacterial consortium associated with AOD, consisting mainly of Brenneria goodwinii, Gibbsiella quercinecans, Rahnella victoriana, and Lonsdalea britannica, which produce degrading enzymes that contribute to phloem necrosis and the development of stem bleeds and bark cracks. However, the role of other pathogens, such as fungi, cannot be ruled out, but instead could be contributory. The potential involvement of xylophagous insects is also being studied, particularly Agrilus biguttatus, which, although, frequently associated with the disease, has not been conclusively demonstrated to act as an active vector of the bacteria. Currently, disease management requires integrated approaches, including monitoring and other forestry strategies to increase forest resilience. Given the phenomenon’s complexity and the risk of the future expansion of that bacterial consortium, further research is necessary to understand the dynamics and to develop effective containment strategies of AOD-associated bacteria.

Acute Oak Decline (AOD), a bacterial disease previously known in Northern and Central Europe, has recently been reported in Salento (a Mediterranean coastal region of Southern Italy), where holm oak trees exhibiting AOD-like symptoms have tested positive for infection with AOD-related bacteria such as Brenneria goodwinii and Gibbsiella quercinecans. Sampling symptomatic trees, strains BLEC23 (B. goodwinii) and GSAC47 (G. quercinecans) were isolated and identified by partial 16S rRNA and gyrB gene sequencing. Pathogenicity tests demonstrate that these bacteria induce wood necrosis when inoculated in excised branches, providing details for the etiology of AOD in Italy. Phylogenetic analysis indicated a substantial genetic similarity between the Italian strains and those found in various European and non-European countries. These findings leave a space open to the possibility that the bacteria involved in AOD are much more widespread in Europe than the findings indicate, but that their presence is frequently hidden.

The impact of Xylella fastidiosa (Xf) subsp. pauca on the environment and economy of Southern Italy has been devastating. To restore the landscape and support the local economy, introducing new crops is crucial for restoring destroyed olive groves, and the almond tree (Prunus dulcis Mill. D. A. Webb) could be a promising candidate. This work focused on the resistance of the cultivar “Filippo Ceo” to Xf and evaluated its physiological and molecular responses to individual stresses (drought or pathogen stress) and combined stress factors under field conditions over three seasons. Filippo Ceo showed a low pathogen concentration (≈10³ CFU mL⁻¹) and a lack of almond leaf scorch symptoms. Physiologically, an excellent plant water status was observed (RWC 82–89%) regardless of the stress conditions, which was associated with an increased proline content compared to that of the control plants, particularly in response to Xf stress (≈8-fold). The plant’s response did not lead to a gene modulation that was specific to different stress factors but seemed more indistinct: upregulation of the LEA and DHN gene transcripts by Xf was observed, while the PR transcript was upregulated by drought stress. In addition, the genes encoding the transcription factors (TFs) were differentially induced by stress conditions. Filippo Ceo could be an excellent cultivar for coexistence with Xf subps. pauca, confirming its resistance to both water stress and the pathogen, although this similar health status was achieved differently due to transcriptional reprogramming that results in the modulation of genes directly or indirectly involved in defence strategies.

Acute Oak Decline (AOD) is a complex and rapidly progressing disease affecting several Quercus species across Europe. While previously reported in Quercus ilex in Italy, this study provides the first evidence of AOD symptoms and associated bacterial infection in Quercus coccifera (kermes oak). Symptomatic trees were identified in a Mediterranean forest in southern Italy, and bacterial isolation, qPCR detection, and 16S rRNA sequencing confirmed the presence of Brenneria goodwinii and Gibbsiella quercinecans. Phylogenetic analyses clustered the isolates closely with known AOD-related strains. Pathogenicity tests on excised Q. coccifera branches demonstrated that both bacteria induced wood necrosis and external exudates consistent with natural symptoms, confirming their virulence. These findings expand the known host range of AOD-related bacteria and highlight the potential threat to Mediterranean oak ecosystems. Early detection and monitoring of Q. coccifera decline are essential to inform conservation strategies and forest management practices aimed at mitigating AOD spread.

This is the first worldwide report of X. fastidiosa (Xf) subsp. pauca on Castanea sativa and the first characterization of Xf infection on this species. Plants located in three sites (in a long-term affected area in Apulia) were monitored for symptoms and bacterial concentrations in spring and summer, while microscopic analyses were performed to evaluate the pathogen distribution in the xylem vessels. All chestnut plants appeared asymptomatic but the Xf subsp. pauca strain “De Donno” was present at low concentrations (1.14 × 10⁴ and 1.56 × 10³ cfu mL⁻¹ in April and June) and with a low incidence (respectively, 38% and 30%). The FISH-CLSM (Fluorescent In Situ Hybridization—Confocal Laser Scanning Microscope) analysis showed evident Xf occlusions but in a low percentage (9.2 ± 3.4%); these data can explain the lack of symptoms on the canopy. Furthermore, through a bibliographic analysis it emerged that Philaenus spumarius and Neophilaenus campestris, two Xf vectors present in Europe, are suckling feeding insects on chestnut trees and could be involved in the bacterial transmission to this species. Asymptomatic Xf host species can play a considerable role in new outbreak emergence or in the expansion of existing ones. So, it is essential to identify them to plan more effective monitoring activities.