The role of pathogens, including oomycetes, in long-term ecosystem development has remained largely unknown, despite hypotheses that pathogens drive primary succession, determine mature ecosystem plant diversity, or dominate in retrogressive, nutrient-limited ecosystems. Using DNA sequencing from roots, we investigated the frequency and host relationships of oomycete communities along a 120 000 year glacial chronosequence. Oomycetes were frequent in early successional sites (5 – 70 yrs), occurring in 38 – 65% of plant roots, but rare (average 3%) in all older ecosystems (280 yrs and older). Oomycetes were highly host specific, and more frequent on plant species that declined most strongly in abundance between ecosystem ages. In contrast, oomycetes were not correlated with plant abundance or plant root traits associated with retrogression. The results support the importance of root pathogens in early succession, but not thereafter, suggesting root pathogen-driven dynamics may be important in driving succession but not long-term diversity maintenance.