While host responses to microbial colonization are extensively explored in evolutionarily young land plant lineages like angiosperms, we know relatively little about plant-pathogen interactions in earlier diverging land plants. We studied the response of the early divergent liverwort Marchantia polymorpha to infection with the oomycete pathogen Phytophthora palmivora. We uncovered a robust response to oomycete colonization in Marchantia that consists of conserved land plant gene families. Macroevolutionary comparisons of host infection responses in Marchantia and the angiosperm Nicotiana benthamiana revealed a shared set of orthologous microbe-responsive genes that include members of the phenylpropanoid pathway. The Marchantia transcription factor MpMyb14 activates the phenylpropanoid (flavonoid) biosynthesis during oomycete infection. MpMyb14 mediates the accumulation of anthocyanin-like pigments and enhanced resistance to infection. The Marchantia response to oomycete infection displays evolutionarily conserved features indicative of an ancestral pathogen deterrence strategy centered on phenylpropanoid-mediated biochemical defenses. The full manuscript can be found here: Carella et al. (2019), Current Biology, https://doi.org/10.1016/j.cub.2019.05.078