The Potential Role of Acidic Soils in the Low Incidence of <i>Phytophthora</i> in Belgian Beech Forests

ABSTRACT Phytophthora monitoring was carried out across 20 beech stands in southern Belgium. Twenty dominant or co‐dominant trees were observed at all 20 sites. In 2022, more than half of these trees displayed crown transparency and branch dieback. In each site, soil samples were collected from three trees representing different levels of decline to assess the presence of Phytophthora in the fine roots. Baiting tests were performed in 2019 (9 sites) and 2021 (20 sites). Among all baited samples, P. cinnamomi was the only Phytophthora species recovered. It was found in a single site (site 14, Mormont) located in a very humid area, in 2021, and its presence was confirmed in 2025. To our knowledge, this is the first report of this species in beech forests in Belgium. All baiting results at the other 19 sites were negative. Baiting tests conducted by artificially infecting the soils from eight of the 20 sites with various Phytophthora species associated with beech decline in Europe were negative, except for P. cinnamomi , showing a different behaviour of this species according to the characteristics of the soils studied. Replacing demineralised water with filtered river water or a seaweed solution for baiting tests, both of which increase the pH of the soil suspension, restored the infection potential of the Phytophthora species studied, as reflected by necrosis on baited leaves. However, when soils that had previously tested negative using demineralised water were reanalysed with filtered river water or a seaweed solution as baiting media, the baited leaves remained symptomless. These trials demonstrate that soils with a low pH limit the infective potential and the occurrence of Phytophthora . They also highlight the higher risk represented by P. cinnamomi for Belgian beech forests growing in acidic soils.