| Summary: | Plants harbor a diverse community of microbes, whose interactions with their host and each other can influence plant health and fitness. While microbiota in plant vegetative tissues has been extensively studied, less is known about members of the seed microbiota. We used culture-based surveys to identify bacteria and fungi found in the seeds of the model tree, <i>Populus trichocarpa</i>, collected from different sites. We found that individual <i>P. trichocarpa</i> seeds typically contained zero or one microbe, with common taxa including species of <i>Cladosporium</i>, <i>Aureobasidium</i>, <i>Diaporthe</i>, <i>Alternaria</i>, and <i>Pseudomonas,</i> a bacterium. <i>Pseudomonas</i> isolates were associated with seed mortality and were negatively associated with the occurrence of fungal isolates within <i>Epicoccum</i>, <i>Alternaria</i>, and <i>Aureobasidium</i> from the same seed. Next, we conducted an inoculation experiment with one of the isolated seed microbes, <i>Pseudomonas syringae pv. syringae</i>, and found that it reduced seed germination and increased seedling mortality for <i>P. trichocarpa</i>. Our findings highlight common fungi and bacteria in the seeds of <i>P. trichocarpa</i>, prompting further study of their functional consequences. Moreover, our study confirms that <i>P. syringae pv. syringae</i> is a seed pathogen of <i>P. trichocarpa</i> and is the first report that <i>P. syringae pv. syringae</i> is a lethal seedling pathogen of <i>P. trichocarpa</i>, allowing for future work on the pathogenicity of this bacterium in seedlings and potential antagonism with other seed microbes.
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