Downy mildew, caused by Plasmopara halstedii (Farl.) Berl. and de Toni, is a very serious disease of sunflower. Studies on the evolution of sunflower downy mildew conducted in 1996 and in 1997 showed that this disease was spreading to new sunflower-growing areas. In 1996, among 146 fields inspected, 77 (53%) were found to be infested with downy mildew. The Gharb region had the highest proportion of fields affected (60%), followed by Saïs (50%) and Loukous (41%). In 1997, 33% of the fields

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... ed were infested. Again, the Gharb was the most frequently infected region with 39%, followed by Saïs (32%) and Loukous (17%), the last a region where downy mildew affects fewer fields. The incidence of mildew in sunflower plants in most of the regions averaged about 3% in 1996, and exceeded the values for 1997, when it was 5%. The disease is most often randomly distributed in the field. The surveys conducted with a sample of a hundred farmers showed that the lack of rotation and the utilisation of chemically untreated seeds of sensitive varieties favoured the disease.