Forest decline induced by Phytophthora is a global phenomenon that affects many tree species. In central Europe, the health of Quercus robur has been declining due to various biotic and abiotic factors, since the 2000s. In drought-prone areas in the Mediterranean region, forests of Q. ilex show maintained tree mortality since the 1980s. The efficacy of potassium phosphite, applied as a foliar or trunk spray at a range of concentrations between 0 and 35%, was investigated over several years in four field trials with mature oaks. Specifically, Experiments 1 and 2, conducted on Q. robur in Poland indicated that aerial and trunk spray of 35% phosphite improved the crown condition of unhealthy trees. The efficacy of treatments was not dependant on the initial tree health condition, although its effects started earlier in trees that had lower crown transparency. Treatments did not alter the abundance and composition of edaphic and endophytic bacteria of trees. Moreover, aerial spray enhanced the N, P and Mg leaf content and induced the formation of fine roots without compromising the secondary growth of trees. In Experiment 3, conducted in Spain on Q. ilex, foliar spray did not improve the crown condition of trees. However, foliar spray of 0.56% phosphite induced twig growth in treated trees located on a southern exposure. Reduction of Phytophthora inoculum by the use phosphite was not observed. Our study provides the foundation for the development of a cost-effective management tool for Phytophthora in Europe and motivates future research into the optimization of treatments in Q. ilex and Q. robur.