Downy mildew is a most harmful sunflower disease in Serbia. In years favorable for disease development, up to 50% of plants may be infected, the yield of systemically infected plants being reduced by about 50%. After the introduction of domestic hybrids resistant to the causal agent, the disease occurs sporadically and causes limited damage. The causal agent of the disease is the fungal species Plasmopara halstedii (syn.: P. helianthi). Symptoms of the disease are twofold: systemic infection and local infection. Systemic infection occurs on roots, seedlings and young plants. Local infection occurs on mature plants, as secondary infection. Typical symptoms of the disease occur on the reverse side of the leaf. Those are the mycelium, conidiophores and conidia which form a thick white layer. The systemically infected plants have a retarded growth rate. They form small heads which are frequently sterile. P. halstedii oospores overwinter and persist in infected harvest residues and seeds. During growing season, the fungus proliferates by developing conidia on the surface of infected plant parts. The species P. halstedii has a large number of physiological races. Control measures include agrotechnical and chemical ones. Chemical measures include seed treatment with metalaxyl-based fungicides.