Cork oak (Quercus suber L.) is an evergreen tree native to SW Europe and NW Africa. It covers 2·106 ha in the western Mediterranean basin, forms heterogeneous forest ecosystems, and represents an important source of income via cork production. While in Iberia, Italy, Tunisia, and Algeria, drought and several endemic pathogens have been associated with cork oak decline (Moricca et al. 2016; Smahi et al. 2017), in Morocco there is no evidence, apart from overgrazing and human intervention (Fennane and Rejdali 2015), of a pathogen associated with oak decline. In December 2019, extensive dieback and mortality of 60-year-old cork oak trees were observed in a natural stand of ∼150 ha located 5 km east of Touazithe, in Maâmora forest, Morocco (34°13′38″N, 6°14′51″W - 87 m a.s.l.). Two years before, Q. suber seedlings from a local nursery were planted to increase tree density. Symptoms in trees and planted seedlings included chlorosis, reddish-brown discoloration of the whole crown, and dieback starting in the upper crown. Root rot and lack of fine roots were observed. Tree mortality was estimated at ∼30%, and disease incidences of trees and seedlings were 45 and 70%, respectively. A Phytophthora species was consistently isolated from the rhizosphere of three symptomatic trees randomly selected at the site using leaves as bait (Jung et al. 1996). On carrot agar, Phytophthora colonies were uniform and cottonwool-like. Sporangia were typically terminal, with ovoid and obpyriform shape, mostly papillate, 30.7 ± 4.7 µm long and 22.7 ± 4.1 µm wide. Oogonia were produced in single culture, and they were globose to subglobose, elongated to ellipsoid, 32.1 ± 2.9 µm in diameter and 46.1 ± 4.8 µm long. Oospores were usually spherical, thick walled, and 28.1 ± 2.4 µm. Antheridia were paragynous, mostly spherical, and 12.2 ± 1.4 µm. Isolates had minimum and maximum temperatures of 5 and 30°C, respectively, and a growth optimum at 20°C. Apart from the small size of sporangia, features were typical of Phytophthora quercina Jung. The identity of a representative strain (TJ1500) was corroborated by sequencing the ITS and mitochondrial cox1 gene regions, and BLAST search in GenBank showed 100% homology with sequences of the extype culture of P. quercina (KF358229 and KF358241 accessions, respectively). Both sequences of the representative isolate were submitted to GenBank (accessions OP086243 and OP290549). The strain TJ1500 is currently stored in the culture collections of the Mendel University in Brno and the University of Sassari. Its pathogenicity was verified and compared with a P. cinnamomi strain in a soil infestation test with 1-year-old cork oak seedlings (Corcobado et al. 2017). Five months after inoculation, the symptoms previously described in the field were observed in the seedlings, and fine root weight of plants inoculated with the TJ1500 strain and P. cinnamomi was reduced by 19 and 42%, respectively, in relation to noninoculated controls. The pathogen was reisolated from the necrotic roots, thus fulfilling Koch’s postulates. So far, P. quercina has been reported associated with chronic mortality of cork oak in new plantations in Spain (Jung et al. 2016; Martín-García et al. 2015) and natural forests in Italy (Seddaiu et al. 2020). To our knowledge, this is the first report of P. quercina in Morocco. Given that Morocco is an important cork-producing country, our finding warns about the risk this pathogen poses to Q. suber and other North African oaks.