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.

[2] Plasmopara halstedii is an obligate biotroph that attacks the flowering plants of the family Asteraceae, found to infect the genus’ Helianthus, Bidens, Artemisia, and Xanthium.

The pathogen has the strongest impact on Helianthus, degrading flower yields in the species H. argophyllus, H. debilis, H. petiolaris and H.

Xanthium strumarium,[4] the common cocklebur, and Ambrosia artemisiifolia,[5] or ragweed, have been shown to act as significant wild hosts.

[8][9] Systemically infected sunflower plants may have some degree of stunting and the leaves show pale green or chlorotic mottling which spreads along the main veins and over the lamella.

Plasmopara halstedii is a plant pathogenic oomycete, capable of overwintering in soil due to survival structures called oospores.

Between long-surviving resting spores and high levels of secondary inoculum, P. halstedii can infect from 50% to 95% of sunflower yields in a single season.

[16][17] Seed treatment has been shown to be effective in controlling the disease, as the establishment of P. halstedii in an area of soil is nearly irreversible.

[19] However, some strains of P. halstedii have begun to show resistance to metalaxyl-based fungicides, reported in multiple countries.