The obligate biotrophic Oomycete, Plasmopara halstedii, causal agent of sunflower downy mildew, is capable of producing new pathogenic races over time. Although changes in the P. halstedii race composition were reviewed for the first time in 2007, since then the pathogen has continued to change its virulence character dramatically. There was a need, therefore, to update information on pathogenic diversity of P. halstedii by making accounts of the temporal and spacial changes in the pathogen populations in North and South America and Europe. This paper, based on current publications and personal communications, attempts to present an accurate overview of races in Europe and Americas for the last 7 years.

Collections of the causal agent of sunflower downy mildew (Plasmopara balstedii) (PH) from several European and North and South American countries were used to inoculate a series of sunflower inbred lines to differentiate races. Race 3 was identified from Argentina; race 4 from France, Hungary, and Bulgaria; race 6 from Canada and France; and race 7 from Argentina. This is the first report of race 3 in South America and of race 4 in Europe, and is the first identification of races 6 and 7. Reactions of USDA lines RHA‐274, RHA‐325, and DM‐2 differentiate races 2, 6, and 7, with RHA‐274 resistant to all three. Thirty‐three commercial hybrids from 11 countries were tested; 70 % were resistant to race 2, only 27 % had resistance to race 6, and none were resistant to race 4. Surveys to determine the relative proportions of the various races within each country are needed to assess their potential impact. The USDA lines HA‐335, HA‐336, HA‐337, HA‐338, HA‐339, RHA‐340, HA‐R4, and HA‐R5 are resistant to all seven mildew races and could be utilized for the production of PH resistant hybrids. For the short term, it appears that seed treatment with metalaxyl may be the most effective control.

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This review summarises the progress in research on sunflower downy mildew as reported in publications of the past 10 to 15 years, the period since the last comprehensive review on Plasmopara halstedii. Particular attention is paid to subjects that showed much progress and may be of particular interest to sunflower pathologists, mycologists or molecular biologists. Accordingly, single sections are devoted to the problems of taxonomic and phylogenetic aspects, host specificity, the hostpathogen interaction including resistance phenomena, as well as epidemiology and disease management. Reflecting the progress achieved in some fields and illuminating the deficits in others should stimulate the reader's interest in this very significant pathosystem.