Wild olive (Olea europaea var. sylvestris) is a sclerophyllous drought-tolerant tree native to the Mediterranean basin, which is used as a hardy and disease-resistant rootstock on which scions of cultivated olive varieties are grafted (Breton et al., 2006). In Sardinia, Italy, wild olive is a significant component of the agroforestry landscape, and its ancient origin is proven by the presence of several millennial trees. In March 2022, extensive dieback and mortality of wild olive trees were observed in an area of c. 200 ha, located 5 km south from Paulilatino in central Sardinia (Fig. 1). Three separate plots were examined and in each the trees showed severe leaf chlorosis, wilting or defoliation of the whole crown (Fig. 2). Often these symptoms were associated with root rot and necrotic lesions on the feeder roots (Fig. 3). In the investigated sites, estimated tree mortality and disease incidence were 60 and 80%, respectively. Rhizosphere soil was sampled from a total of 16 unhealthy trees and subjected to a baiting technique, using fresh Ceratonia siliqua leaves as baits (Perez-Sierra et al., 2022). Isolations were also made by plating small fragments of root phloem tissue from lesion margins onto Phytophthora selective medium (SMA) (Brasier et al., 2005). A homothallic Phytophthora species was consistently isolated from 14 of the sampled trees. An additional Phytophthora species was detected from the rhizosphere of four diseased trees. Developing colonies were transferred onto carrot agar, and DNA extracted for amplification and sequencing of ITS and cox1 genes. A BLAST search in GenBank showed 99–100% identity with sequences of the ex-type cultures of P. bilorbang (CBS131653) and P. pseudocryptogea (CBS139749). Sequences of a representative isolate for each species were submitted to GenBank (Accession Nos. OP801844, OP801845, OP856579 and OP856580). The pathogenicity of both Phytophthora species was verified using six-month-old wild olive seedlings and the soil infestation method (Perez-Sierra et al., 2022). Three weeks after inoculation with P. bilorbang and P. pseudocryptogea, all seedlings showed wilting and dieback, and the fine root weight of inoculated plants was reduced by 68 and 42%, respectively, in relation to non-inoculated controls (Fig. 4). Both species were re-isolated from necrotic roots, thus fulfilling Koch's postulates. This is the first report of P. bilorbang on wild olive worldwide. However, this species was recently detected in Calabria, Italy, causing root rot on 15-year-old cultivated olive trees originating from Sardinia (Santilli et al., 2020). In contrast, P. pseudocryptogea was previously reported on wild olive in Spain together with other Phytophthora species (González et al., 2019). Further investigations are required to better understand the epidemiology of the disease and other possible factors involved in the decline. A consequence of this damaging new disease is that further use of wild olive as a source of rootstock for grafting olive varieties could be limited. We thank the local municipality of Paulilatino for their logistic support in the field surveys.