Assessing the Efficacy of Large‐Scale K‐Phosphonate Treatment to Mitigate Cork Oak Decline Associated With Phytophthora cinnamomi in Italy

ABSTRACT Cork oak forests are of significant environmental and socioeconomic importance in the Mediterranean basin. However, these biocenoses are facing a progressive decline due to multiple biotic and abiotic factors. Among these, Phytophthora species, particularly P. cinnamomi , remain among the primary drivers of decline. Mitigation of the impact of these pathogens relies on good practices, including reducing the passive movement of inoculum and increasing tree resilience. The latter can be achieved through various strategies, including the use of phosphonate salts, specifically potassium phosphonate. The present study aimed to assess changes in crown decline severity following large‐scale potassium phosphonate treatment by trunk injection in mature cork oak trees growing in decline areas associated with P. cinnamomi in a natural forest in Italy. Treatment response was assessed in 851 mature trees monitored for up to 24 months following potassium phosphonate treatment. One year after potassium phosphonate application, DS decreased by 13% and 7% in the two monitored decline areas, respectively, while transition outcomes differed significantly according to initial decline class. Trees with intermediate decline severity (Classes 1–2, defoliation up to 50%) showed the most favourable crown‐condition responses, with improvements still observed at 24 months in decline area 1. Trees with defoliation exceeding 50% (Classes 3–4) showed a less consistent response than those in Classes 1–2, indicating increased variability in treatment outcomes beyond intermediate levels. Although some individual trees improved, these improvements were often balanced by a comparable number of trees exhibiting increased decline severity. In both untreated reference plots, decline severity increased over time. These findings provide field‐scale evidence of favourable crown‐condition changes following operational potassium phosphonate treatment and support further evaluation of this approach within integrated management strategies for cork oak decline associated with P. cinnamomi .