Abstract: In Spain, natural and managed oak woodlands are severely affected by decline caused by the root rot oomycetes Phytophthora cinnamomi and Pythium spiculum. As the use of phosphite is prohibited in Spain, trunk injections of Fosetyl-aluminium (Fos-Al) were checked for disease management. Seminatural woodlands (dehesa) dominated by holm or cork oaks, were selected for treatments. For each woodland, 60 oaks in three defoliation classes (asymptomatic, slight, and moderate defoliation) were selected for trunk injection with 4% of commercial Fos-Al or water (controls), and periodically checked for defoliation and presence of pathogens in roots and rhizosphere. Three years after treatments, Fos-Al significantly decreased defoliation in treated oaks in comparison with water-injected controls, regardless of the initial defoliation class considered. Changes in inoculum density in the rhizosphere or presence of pathogens in roots were not significantly dependent on treatment. However, a trend to a lower presence of pathogens in roots of treated oaks was observed with increasing soil inoculum densities. Fos-Al was experimentally proved to protect oaks against root rot caused by both pathogens, even exhibiting a therapeutic effect on preexisting infections. Consequently, this effective measure should be considered as part of the integrated management of oak root rot.