New races of the sunflower downy mildew pathogen (Plasmopara halstedii) in Europe and North and South America.
Collections of P. halstedii 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 3. A total of 33 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. It is suggested that the USDA lines HA-335, HA-336, HA-337, HA-338, HA-339, RHA-340, HA-R4 and HA-R5 which are resistant to all 7 mildew races could be utilized for the production of P. halstedii resistant hybrids.