One of the actual problems of lettuce is downy mildew. This disease is caused by the oomycete Bremia lactucae. The frequent use of chemicals and resistant lettuce varieties give rise to high costs, risks for the wellbeing of both environment and consumers, and fast development of resistance of the pathogen. Cultivation in greenhouses provides the opportunity of reducing the disease pressure by adapting the climate to create circumstances less suitable for the pathogen. Therefore, sufficient knowledge of the epidemiology of B. lactucae is needed.



Experiments in the lab of phytopathology of the University of Ghent and field trials in the greenhouses of Inagro, PCG and PSKW indicated that Bremia lactucae has different climatological needs during his life cycle:

• Spores will germinate during the morning and the germination is stimulated by leaf wetness, requiring a relative humidity of greenhouse air above 90%.
• During the night sporulation can occur when the relative humidity reaches values above 85%.
• Early in the morning spores can be released. This release is triggered by the first light and favoured by rising temperature and decreasing relative humidity.

From these findings we can conclude that lowering the relative humidity in the greenhouse can reduce both the sporulation and germination of the spores and thus diminish the disease pressure. These findings are confirmed by the latest field trials of which the results are represented in the figure below.

• The number of infected plants in greenhouses where the relative humidity did not reach 90% was five to six times lower than in greenhouses with a higher relative humidity.
• The symptoms of the diseased plants in the dryer greenhouses were less severe than those in the other greenhouses.
  
  

The ultimate purpose of this project is to present a flowchart with advice towards the breeders, both concerning adaptation of the climate and application of (bio)chemical treatments. For the development of this flowchart more detailed information is necessary. Recent tests in the lab indicate that a relative air humidity of at least 90% during 24 hours enables the pathogen to survive a dryer period of two weeks or longer. As the relative humidity rises again, sporulation might start. This initial period of 24 hours allows spore germination and penetration of the leaf tissue. Preliminary results show that a shorter period might be sufficient, but the exact duration required should be revealed by further research.

Other aspects on which this research will focus are the possible role of oospores as a primary inoculum and the climatological circumstances which favour their formation and germination. In the first part of the project period, oospores were determined in samples from field trials and breeders. As Bremia lactucae is a heterothallic oomycete two mating types exist, which are both needed to fuse for the formation of oospores. Recently, both mating types are identified in pathotypes isolated from diseased plants in Flanders. This excludes the possibility of import of the determined oospores.

The Bremia project is an Agricultural Research project (IWT) coordinated by the Research Centre for Vegetable Production at Kruishoutem and in collaboration with Ghent University- Faculty of Bioscience Engineering, Research Station for Vegetable Production at Sint-Katelijne-Waver and Inagro.