Multiple <i>Phytophthora</i> species associated with declining wild olive trees in Sardinia, Italy

Abstract Wild olive represents one of the most iconic woody plants in the Mediterranean Basin. This slow‐growing evergreen tree is characteristic of sclerophyll vegetation, extremely tolerant to drought, salinity and diseases, thus is commonly used as rootstock for grafting cultivated olive varieties. Since 2022, extensive dieback and mortality of wild olive trees have been observed in Sardinia, Italy. Affected plants showed leaf chlorosis, wilting, defoliation, shoot blight and epicormic shoots, often associated with root rot and necroses on the feeder roots, initially associated with Phytophthora bilorbang and P . pseudocryptogea . A 2‐year study was thereafter conducted to clarify the aetiology of this unusual disease. Using a baiting technique, 10 Phytophthora taxa from three phylogenetic clades (2, 6, 8) were isolated from 53 of 87 rhizosphere soil samples collected, including P . asparagi , P . bilorbang , P . crassamura , P . inundata , P . kelmanii , P . oleae , P . pseudocryptogea , P . sansomeana and P . syringae . Additionally, some isolates that showed several polymorphisms in rDNA internal transcribed spacer (ITS) sequences compared with closely related known species are reported here as P . taxon paulensis. Pathogenicity tests on 1‐year‐old wild olive seedlings showed that nine out of 10 Phytophthora taxa significantly reduced root length of inoculated seedlings compared to controls. P. inundata and P . oleae caused the greatest reduction, while root length of seedlings inoculated with P . kelmanii did not differ from the controls. This study provides insights into the role of Phytophthora taxa in the wild olive decline in Sardinia, indicating the urgent need to expand monitoring and implement effective management strategies to mitigate the disease.