Early stages of infection of resistant and susceptible sunflower seedlings by three races of Plasmopara halstedii

Resistance in sunflower (Helianthus annuus L.) to downy mildew (Plasmopara halstedii (Farl.) Berl. et de Toni) is conditioned by a series of Pl. genes. Races 1 (European race), 2 (Red River race), and 3 (a new North American race encountered in the Red River valley in 1980) formed similar infection structures on inoculated seedlings whether susceptible or resistant to races 1 and 2; all cultivars tested were susceptible to race 3. The zoospores encysted and produced germ tubes and appressoria on the roots and hypocotyls. Penetration occurred and mycelium and haustoria developed in both compatible and incompatible host–pathogen combinations. Resistance did not appear to involve a hypersensitive reaction occurring prior to penetration.