Thesis title: Investigations and aetiological analyses on serious decline of olive trees in Apulia due to fungal microorganisms, and confusable with OQDS (Xylella fastidiosa)
Olive (Olea europaea L.) represents a key crop for the Mediterranean Basin, with a central economic, cultural and ecological role, particularly in southern Italy. Over the last two decades, olive groves in Apulia have been severely affected by a widespread decline phenomenon, initially attributed exclusively to Olive Quick Decline Syndrome (OQDS), associated with the bacterium Xylella fastidiosa subsp. pauca (Xfp). However, increasing field observations and early investigations suggested that additional biotic factors might be involved in disease expression and progression.
This thesis, conducted as part of a four-year PhD program, aims at clarifying the contribution of fungal communities to olive decline in Apulia, their interaction with Xfp, and the influence of abiotic stressors on host–pathogen relationships. A large-scale survey was carried out between 2022 and 2024 across the region, and specifically in ten distinct areas, integrating systematic field sampling with culture-based isolations, molecular diagnostics and multi-locus phylogenetic analyses. Pathogenicity trials, including mono- and multifactorial experiments, were conducted to assess fungal virulence under non-stressed and stress-conditioned scenarios.
Survey results revealed a high prevalence and frequent co-occurrence of Xfp with fungi belonging mainly to the Botryosphaeriaceae and Apiosporaceae families in symptomatic plants. Both fungal families could be considered important actors, even though Botryosphaeriaceae seem to be the major contributors in the symptoms overlapping with those of ODQS. These findings support the hypothesis of a complex and multifactorial disease system affecting olive trees in Apulia.
Within the Botryosphaeriaceae, Diplodia seriata emerged as the most recurrent species across the surveyed areas, yet it behaved as a low-virulence pathogen under non-stressed conditions. Pathogenicity trials demonstrated that its pathogenic potential is strongly modulated by environmental factors, markedly increasing under water stress and, in particular, under combined water and thermal stress.
In contrast, Neofusicoccum mediterraneum, although detected at a comparatively low frequency in the investigated areas, consistently proved to be the most virulent species in pathogenicity assays, inducing severe symptoms regardless of plant water status.
Neofusicoccum stellenboschiana showed an intermediate virulence compared with the other Botryosphaeriaceae, while being widely distributed across the surveyed areas. This combination of recurrence and moderate pathogenicity suggests that its role in olive decline may be underestimated under non-stress conditions and deserves further investigation under abiotic stress scenarios.
Metagenomic analyses, introduced and partially developed in this thesis, will provide an additional, unbiased insight into fungal community composition and recurrence, complementing traditional diagnostic approaches.
In conclusion, this thesis provides novel insights into the olive pathosystem by elucidating the distribution and association of fungal communities with olive decline syndromes. The findings highlight the opportunistic nature of these micro-organisms, specifically their ability to undergo a pathogenic transition from an endophytic lifestyle to a virulent state when environmental stressors compromise the host’s immune system. Overall, this work supports a shift from a single-pathogen paradigm towards a multifactorial framework in which microbial interactions and environmental stress play a central role in olive decline in Mediterranean agroecosystems.