Abstract
Virus-like particles were observed in five North Dakota isolates of Plasmopara halstedii race 2, the causal agent of sunflower downy mildew. Virus-like particles were observed in fungal mycelium within sunflower seedlings showing typical symptoms of systemic infection as well as from seedlings displaying "twisted leaf syndrome." Virus-like particles were never observed in sunflower cells. The particles were isometric in shape and measured 23–26 nm in thin sections and 32 nm after phosphotungstic acid staining. Virus-like particles occurred dispersed throughout the cytoplasm and in membrane-bound organelles. Both crystalline and noncrystalline arrays were common. The particles were observed in haustoria, intercellular hypha, sporangiophores, zoosporangia, and oospores. Four species of double-stranded RNA were detected in sunflower tissue infected with P. halstedii; none were detected in healthy sunflower tissue. This is only the second report of an isometric, virus-like particle occurring in a member of the Peronosporales. Key words: mycovirus, downy mildew, Helianthus annuus.
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