Overwintering seeds as reservoirs for seedling pathogens of wetland plant species

Seed germination and seedling establishment are central to the distribution and abundance of plant species in wetlands. While fungal and oomycete pathogens are known to affect seed viability and emergence, relatively little is known about which fungi and oomycetes are associated with seeds in the soil or how these species affect seeds and seedlings. We characterized the fungi and oomycetes associated with overwintering seeds in wetlands and determined their potential to influence seed germination and subsequent seedling mortality. Fungi and oomycetes did not affect seed germination, despite the isolation of high frequencies of known seed and seedling pathogens in the fungal genera Alternaría, Peyronellaea, Epicoccum, and Fusarium. However, many of the most frequently isolated fungal species from overwintering seeds were highly virulent to seedlings. While both native and nonnative plant species were tested, we did not observe consistent differences in either seed germination or seedling susceptibility based on the invasive status of plants tested, contrary to what we expected given several established hypotheses for invasive success. The high seedling virulence of fungi from overwintering seeds coupled with the differential abundance of some of the more pathogenic fungi among seeds of different plant species, led us to the conclusion that the fungal pathogens that colonize seeds in the seed bank over winter are likely to strongly impact subsequent seedling establishment in wetlands the following spring despite not reducing overwintering seed germination in the seed bank or differently effecting invasive plant species.