Biologische beheersstrategie van Valse Meeldauw in zonnebloem

At the moment downy mildew is the most urgent problem in sunflower cultivation in the Netherlands. Because sunflower is one of the basic products in the organic flower bouquets it is important to solve this problem. Downy mildew is caused by the oomycete Plasmopara halstedii. The initial infection comes from the soil where oospores can survive for up till 10 years. For now the only way for producers of organic sunflowers to control the mildew is to use a very broad crop rotation or use fresh land every year. The aim of this project is to test a biological control strategy in a practical situation in order to successfully control downy mildew originating from the soil. The first strategy tested was the use of a compressed plug of potting soil. Young sunflower seedlings are most susceptible for downy mildew infections and the use of a plug could prevent infection during the early growing phase. Unfortunately, due to the lack of infection in the field trials no conclusions on the effects of the use of these plugs can be drawn. The second strategy was to determine the risk of infection by analysing the soil of the field for infection with Plasmopora before sowing sunflower. Different sampling methods were tested in a bioassay. However, the infection percentage was to low to draw any conclusions. The third strategy was to use biological soil desinfestation as a control strategy for downy mildew. The principle of this method is to create a rapid oxygendeficiency in the soil which a lot of micro organisms do not survive. The oxygendeficiency is created by ploughing fresh grass through the soil and covering the plot with plastic non permeable for oxygen. This method appeared to be very effective in controlling downy mildew and is recommended to growers.