Plasmopara halstedii is a plant pathogen infecting sunflowers. The species is one of many pathogens commonly referred to as downy mildew. P. halstedii originated in North America. P. halstedii oospores produce a thin wall which are resistant structures, sexually produced that are essential for its continuation. After entering an area, the eradication of the pathogen is difficult due to the formation of oospores, which can remain viable in soil for many years.