In the last decades, tree decline in European beech forests has been related to extreme climatic events and with the activity of parasitic oomycetes such as Phytophthora species. In Spain, little is known about the association of beech decline and soil oomycete composition. We hypothesized that the weakening of beech trees is associated with the activity and proliferation of parasitic oomycetes. We studied the rhizosphere oomycete community of large beech trees located in one of the southernmost beech forests in Europe. From soil samples collected in the rhizosphere, we used a metabarcoding approach to explore the oomycete community associated to asymptomatic and declining trees. We also studied fine root parameters in both groups of trees. A total of 99 oomycete amplicon sequence variants (ASVs) were detected. Higher diversity and richness of oomycetes were observed in declining than in asymptomatic trees. The oomycete composition also differed between the two groups of trees. Declining trees showed lower fine root biomass and root density than asymptomatic trees, and root density was negatively correlated with the abundance of Pythium ASV counts. The genus Phytophthora, associated with beech decline in central Europe, was underrepresented in the oomycete community. The results suggest that decline-associated processes in beech trees are related with fine root weakening and loss which probably facilitates the entrance and colonization of opportunistic oomycete microbes. Although members in the genus Pythium do not seem to be a primary factor on beech decline, they probably contribute to the chronic decline of Fagus sylvatica trees.

This work was supported by the SYLVADAPT project (PID2019-107256RB-I00; Ministerio de Ciencia, Innovación y Universidades). Clara Martínez-Arias was mostly funded by the progam “Margarita Salas” of Universidad Politécnica de Madrid (ref. RCMS-22-WOLATK-10-QDCKPJ). David Macaya-Sanz was mostly funded by the program "Atracción de Talento Investigador" of the Community of Madrid (ref. 2019-T2/BIO-12780). Juan Piñeiro was supported by the Ramón y Cajal program from Ministerio de Ciencia, Innovación y Universidades (RYC-2021-033454).

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