Transition of secondary to systemic infection of sunflower with Plasmopara halstedii – An underestimated factor in the epidemiology of the pathogen

Plasmopara halstedii, the downy mildew pathogen of sunflower causes significant economic loss world-wide, mostly through soilborne systemic infection of seedlings. Natural infection of sunflower with P. halstedii was monitored in a sunflower field cultivated for ornamental purpose in soil where no sunflower had been grown before. Local and systemic infections were observed in plants of different developmental stages which were sown in five consecutive field plots between Apr. and Jul. The airborne origin of the infection by zoosporangia was concluded from field history, pathogenic symptoms, time course of infection and microscopic investigation of mycelium distribution in stems. A high potential for transition from local to systemic infection was found, at least in ornamental sunflower cultivation under the typical weather conditions in Central Europe. This questions the paradigm that economically and epidemiologically relevant sunflower downy mildew incidences are only derived from subterranean infections. Airborne secondary infections, as they may occur in all developmental stages and on all organs of the host plant, are responsible for late systemic infection and can play a key role in the production of contaminated seeds carrying the pathogen into the next season.