Moving the “olive line” northward: suggestions from leaf-inhabiting fungi in unmanaged old olive trees in Northern Italy

Climate change has been shifting to higher latitudes and altitudes the suitability for olive tree cultivation. Thus, olive trees already extant at the current edge of the distribution area show possible future scenarios of response to challenging environments. Here, the oldest olive trees in Pavia province (Northern Italy) were surveyed for diversity of leaf-inhabiting fungi in unmanaged conditions. Alternaria alternata resulted ubiquitous and highly represented; multi-locus molecular barcoding on the isolates suggests they belong to the species sensu stricto synonymized with A. tenuissima. The growth profiles of A. alternata strains from old individuals look different from the younger ones, possibly indicating the latter are non-native. For the first time, Trichothecium roseum was found to inactivate necroses caused by Venturia oleaginea and Curvularia spicifera was isolated from olive tree. The frequency of Chaetomium globosum sensu lato suggests it may counteract leaf pathogens such as emerging Pleosporaceae and the opportunistic Sordaria fimicola. Strains from Fusarium graminearum species complex and Schizophyllum commune were isolated as possible commensalists. In conclusion, this survey excluded the presence of severe fungal pathogens in the leaves of old unmanaged olive trees, suggesting such an unusual niche may have shaped peculiar coenoses whose structure and self-balance deserve further investigation.