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The characterization of the seasonal dynamics of endophytic bacteria in beech leaves can be hindered by co-amplification of chloroplast and mitochondrial plant DNA. This study applies established peptide nucleic acid (PNA) clamps to suppress host-derived amplification while resolving bacterial succession across the vegetative season. Chloroplast- and mitochondrion-specific PNAs inverted the proportion of host to bacterial reads, enabled the recovery of bacterial sequence variants, and increased alpha diversity accordingly. Beta-diversity analyses showed that, once host contamination was removed, samples displayed a clear seasonal trajectory. Early-season leaves contained high abundances of Pseudomonas together with taxa likely introduced through plant–insect–microbe interactions. As leaves matured, the microbiome shifted toward a more stable composition dominated by well-established genera. The transition from early transient taxa to the later enrichment of phyllosphere-adapted and nutrient-cycling genera demonstrates that beech leaves host a temporally structured microbiome shaped by leaf development and seasonal environmental stress.