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Physiological Specialization Between Races of Plasmopara halstedii in America and Europe. D. E. Zimmer, Research Plant Pathologist, Agricultural Research Service, U.S. Department of Agriculture, Fargo, North Dakota 58102; Phytopathology 64:1465-1467. Accepted for publication 24 June 1974. DOI: 10.1094/Phyto-64-1465.

Sunflowers, Helianthus annuus, resistant in the United States to downy mildew incited by Plasmopara halstedii, were, without exception, resistant to downy mildew in European trials. However, many lines resistant in trials in Europe were susceptible in the United States. These differences establish physiological specialization within the P. halstedii complex in Europe and America and suggest that appropriate measures be implemented to restrict the chance introduction of the more virulent North American race of P. halstedii into areas where it does not presently occur. Resistance to both the North American and European races of downy mildew is traced to wild H. annuus. Exploration on a wide geographic scale of wild populations of H. annuus for additional genes for downy mildew resistance is suggested.