Overview

Brief Summary

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.

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Wikipedia

Plasmopara halstedii

Plasmopara halstedii is a plant pathogen infecting sunflowers. Plasmopara halstedii originated in North America.

Plasmopara 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.[1]

Detection

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.

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