Abstract
Resistance in sunflower (Helianthus annuus L.) to the widely virulent Red River race of downy mildew, incited by Plasmopara halstedii (Farl.) Berl. et de Toni, was discovered in three inbred lines selected from the cross 953‐88‐3 ✕ ‘Armavirsky 2497.’ All other breeding lines, sunflower introductions, and varieties, including AD 66, CM 90RR, and S‐37‐388RR, which were reported to be resistant in tests in Romania and France, were susceptible. The failure of the breeding lines AD 66, CM 90RR, and S‐37‐388RR to respond the same to downy mildew in our trials as they did in Europe indicates the existence of pathogenic races.Resistance to the Red River race found in HA 61, one of the three resistant lines, is due to a single dominant gene that we have designated Pl2. The Pl2 gene is inherited independently of the gene presumed to be Ra which conditions rust resistance of HA 61. The Pl2 gene is probably synonymous to one of the genes reported to condition resistance of HA 61 to downy mildew in France.
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