The fungus Plasmopara obducens was identified and subsequently declared a notifiable disease.
Crops with the disease confirmed had to be destroyed with no compensation to the grower. In 2005, Defra and the industry agreed that the industry should police itself. For this to happen, growers needed effective techniques for early detection on seed and young propagation material and access to a range of effective control measurements. HDC project PC 230 set out to provide this information.
Lack of disease outbreaks in the UK and Europe meant that the project started using downy mildew of pansy as a model. But Impatiens downy mildew returned in 2007 and was widespread in 2008, so researchers had plenty of material with which to work.
Studies showed that leaf wetness was required for spore production - high humidity was not sufficient. Growers are advised to ensure adequate air circulation around plants to minimise prolonged periods of leaf wetness by better spacing and by increasing ventilation. Overhead watering will aggravate the disease.
Crop safety trials of a comprehensive range of fungicides and types of application were carried out on pansy and Impatiens. The full list is included in the final report.
Apart from maintaining an effective fungicide programme, growers are advised to practice good nursery hygiene and clean debris between crops and at the end of the season to minimise carry-over risks. The project has been extended to include fungicide efficacy trials - results are due next summer.
Horticultural Development Company
For details on all HDC activities, visit www.hdc.org.uk.