Systemic infection of sunflowers byPlasmopara halstediioccurs much more readily through hypocotyls than through roots, and through apical buds than through leaves.Systemic symptoms rarely develop from belowground infections of plants past the four-leaf stage, and from apical bud infections of plants past the eight-leaf stage. Necrotic lesions may occur and the pathogen may sporulate on hypocotyls and roots of plants with no typical systemic symptoms.Infection by sporangia produces local lesions in young leaves; some of these infections may become systemic.Immersion of 3-day-old seedlings in inoculum for 1 min resulted in some infection; highest infection resulted from 3- to 6-h exposure.Systemic infection resulted from apical bud inoculation with suspensions averaging one sporangium per plant.Optimum temperature during inoculation of 3-day-old seedlings was 15°, and during incubation was 20° to 25 °C. Optimum temperature during bud inoculation was 15°; incubation temperatures from 15° to 25 °C gave essentially similar results.