Physiological specialization between races of Plasmopara halstedii in America and Europe.
All the sunflower lines resistant to downy mildew in the USA were also resistant in European trials but many lines resistant in Europe were susceptible in the USA. Measures should be taken to prevent the introduction of the more virulent N. American race of P. halstedii to areas where it does not yet occur. Resistance to both the N. American and European races is traced to wild Helianthus annuus and searches should be undertaken for additional wild resistant material.<new para>ADDITIONAL ABSTRACT:<new para>Trials of 25 sunflower lines showed that those resistant to P. halstedii in the United States were also resistant in Europe, but many lines resistant in European trials were susceptible in the United States, indicating physiological specialization in P. halstedii. The resistance of most of the resistant lines was derived from wild Helianthus annuus.