Wild olive stands out as one of the most iconic evergreen tree species in Mediterranean forests and maquis vegetation. Known for its high tolerance to unfavourable conditions, such as drought, salinity and diseases, wild olives have evolved to withstand different environments and diverse soil types. Its high resistance to biotic and abiotic stressors has made wild olive a valuable rootstock commonly used to graft cultivated olive varieties. Widespread across the whole Mediterranean Basin, in Sardinia (Italy) this species thrives from the coastline to inland areas representing a significant component of the agroforestry landscape, with numerous ancient trees that showcase its remarkable resilience.
However, since 2022, a severe dieback and mortality of wild olive trees have been observed in central Sardinia. Affected trees showed various symptoms including leaf chlorosis, wilting and, in more severe cases, defoliation of the entire crown. These symptoms were often associated with root rot and necrotic lesions on the feeder roots, suggesting the possible presence of a root disease caused by soil-borne pathogens in the genus Phytophthora.
The combination of drone flights with field surveys enabled the detection and mapping of disease outbreaks. Our findings indicated that the disease was spreading rapidly, covering an area of approximately 1,200 hectares and affecting over 10,000 trees.
In our study, we isolated and identified ten Phytophthora taxa from three distinct phylogenetic clades (2, 6, 8), revealing a hidden Phytophthora diversity associated with diseased trees. Pathogenicity tests showed that nine of these taxa significantly reduced the root systems of inoculated wild olive seedlings, with P. inundata and P. oleae being the most aggressive species.
These findings are particularly concerning. Not only do these pathogens pose a serious threat to wild olive woodlands, which represent ecosystems of high ecological value, but their potential spread to cultivated olive varieties could also lead to significant economic losses in olive production.
Our study underscores the importance of using new tools, such as drones and remote sensing, to detect and monitor plant diseases on a large scale. The advancements made so far in understanding both the symptomatology and aetiology of the disease will help us to develop effective management strategies to protect these trees and the broader landscapes they support.
Antonio Deidda, Gabriele G. A. Satta, Andrea Brandano, Camilla Morittu, Davide Mureddu, and Bruno Scanu published this work in Plant Pathology Journal:
TITLE IMAGE: Overview of decline affecting wild olive trees in Sardinia: a drone-captured panoramic view of the affected area, with the red arrow indicating a closer ground-level view of the same location (top-left). A completely defoliated tree and a large necrotic lesion on an exposed root (top-right). A map of Sardinia showing the study area, divided into macroareas (black ovals), with positive (red +) and negative (green –) results for Phytophthora detection. Pie charts represent the isolation frequency of each Phytophthora taxon (bottom-left). Results of pathogenicity tests: a healthy root system from a control seedling compared to root systems inoculated with Phytophthora displaying a clear reduction in root length, absence of fine rootlets, and necrotic lesions (bottom-right).
