Downy Mildew («Plasmapara halstedii») : Importance and Geographical Distribution on Sunflower in Morocco

Abstract

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 surveyed 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.