Downy mildew is caused by a soil borne pathogen that can survive for many years in the soil. Downy mildew of sunflower is caused by P lasmopara halstedii (F arlow) B erlese & de T oni. 130-136. pp. Sunflower Downy Mildew. When frequent rains occur after planting, the pathogen produces swimming spores (zoospores) that infect roots and cause a systemic infection. In 23 of 26 P. halstedii isolates derived from eight countries in Europe, North America and South America, vi … Evolution of the pathogen-host plant relationship, into Plasmopara halstedii (helianthi)-Helianthus annuus L. system Sep 2006 Eight pathotypes of Plasmopara halstedii were screened to investigate the occurrence of virions and the potential viral influence on the pathogenicity of the sunflower downy mildew pathogen. Sunflower Downy Mildew, Fargo (USA), 13-14 January. Sunflower hybrids resistant to downy mildew are available, but new pathogenic forms (pathotypes) of the fungus are being formed in nature, making the use of formerly resistant cultivars in a particular area questionable (Gulya et al., 1991a).