A seed bank pathogen causes seedborne disease: Pyrenophora semeniperda on undispersed grass seeds in western North America

The generalist pathogen Pyrenophora semeniperda is abundant in seed banks of the exotic winter annual grass Bromus tectorum in semiarid western North America and is also found in the seed banks of co-occurring native grasses. In this study. we examined natural incidence of disease caused by this pathogen on undispersed host seeds, that is, seeds that were never directly exposed to inoculum in the seed bank. We determined experimentally that at least 90% of undispersed B. tectorum seeds exhibiting the disease were likely infected after maturity by conidia borne Superficially on the seed-covering structures. The fraction of undispersed seeds exhibiting disease under optimum conditions for infection in the laboratory varied from 0% to 22% for three grass species. Relatively high disease incidence on undispersed seeds for this dry-sporulating fungus was significantly correlated with low mean annual rainfall, dry conditions during the conidial dispersal period in early summer, and high concentrations of soilborne inoculum as evidenced by the presence of high densities of killed seeds bearing fungal stromata in the seed bank. These three variables explained 66% of the variation in seedborne disease incidence for B. tectorum seed collections made in 2005. By dispersing conidia onto host seeds while seeds are still on the plant, this pathogen achieves the potential for targeted dispersal along with seeds of its host. This may be particularly advantageous for P. semeniperda, which has large conidia (phragmospores) that may not be efficiently dispersed beyond the grass canopy boundary layer.