In 1998, a number of Ambrosia artemisifolia individuals located in the vicinity of a sunflower breeding nursery at Bicsérd, south Hungary, showed disease symptoms resembling downy mildew and/or white rust. Sporangia of both Plasmopara and Albugo have been isolated from the affected plants. Microscopical observations and subsequent inoculation experiments revealed the existence of Plasmopara halstedii on the affected A.artemisifolia plants. Following inoculations on a set of sunflower differentials, the Ambrosia isolates of P.halstedii consistently showed a virulence formula of 730 that is equal to pathotype 4.