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sunflower downy mildew
Plasmopara halstedii is a devastating disease of the annual sunflower Helianthus annuus causing downy mildew. The pathogen was first descibed from Eupatorium purpureum by Farlow and its name was subsequently used as well for the downy mildew pathogen on sunflower, although the conspecifity has not been proven yet. Consequently, Novotelnova in Russia described the pathogen on Helianthus as an independant species and named it Plasmopara helianthii (with additional differntiation of subspecies on annuals, perennials and closly related genera).
Biology: The primary infection is usually soilborne, starting from oospores which represent the resting stage of the pathogen. Oospores may be distributed
with seeds of infected plants or are set free from infected tissue of roots and stems. They can rest viable in the soil over years and start germinating with a germ sporangium upon yet unknown signals. The germ sporangium releases zoospores which migrate to the host surface where they encyst, form a germ tube with apressorium and penetrate the host epidermis. When the primaryhaustorium has been established, the biotrophic phase starts.