Approximately 30ha of sweet basil is grown in the UK with about 25% produced under protected conditions. The estimated crop value is "in the order of tens of millions of UK sterling".
Downy mildew in basil is a devastating disease caused by the plant pathogen Peronospora belbahrii, first reported in UK-grown sweet basil in 2010. Outbreaks can cause more than 80% crop loss in field and protected production, with associated financial loss and disruption to the supply chain.
AHDB project PE 024 explored basil downy mildew biology and control options for growers. The work suggested that where the downy mildew pathogen is detected in seed it is carried internally rather than as an external contaminant.
Five products identified with potential to control basil downy mildew were further examined to determine the most appropriate time between fungicide applications. The project found that using Paraat (dimethomorph), Fenomenal (fenamidone + fosetyl-aluminium) (outdoor use only) and Revus (mandipropamid) provided good protection when applied up to 10 days prior to infection. Fubol Gold (metalaxyl-M + mancozeb) failed to control the disease, leading to the discovery that metalaxyl-M resistant isolates of the pathogen are present in the UK.
Growers are urged to regularly monitor crops for disease symptoms, ensure adequate air circulation for protected crops to minimise prolonged periods of leaf wetness, avoid overhead watering, which is likely to aggravate the disease, and consider growing other host crops (agastache, lavender, common sage and catnip) independently of each other. These strategies can lower downy mildew risks and hence associated losses.
For details on all AHDB Horticulture activity, see horticulture.ahdb.org.uk