Abstract: Downy mildew of sunflower [Plasmopara halstedii (Farlow) Berl. et de toni] is one of the quarantined plant diseases in Shanxi province. It was found first in Xinzhou in 1968. At present, it was mainly distributed in the spring and summer-sown area of sunflower in Xin-Ding basin. The sources of primary infection over winter are residues of diseased plant, and infected seeds and soil. The infection rate in the spring-sown sunflower fields was much lower than in the summer-sown fields, but the spore case produced by diseased plants in spring served as an important disease source of reinfection for the summer-sown sunflower. In the years with more rainfall or high moisture, reinfection on sunflower seedlings occured seriously, and the diseased plant rate came up to over 80%.Disease intensity increased with later, repeated or border sowings. The sunflower cultivars used for production now are all susceptible to disease. Fortunately, five high resistant hybrids of oil form have been screened out and supplied for test in production. The seed dressed with Ridomil WP (0.5g a. i./kg), the control effect reached over 94%.