Downy Mildew
Hyaloperonospora parasitica (Peronospora parasitica)
Pathogen
Fungus
Hosts
Oilseed Rape and other brassica species and cruciferous weeds
Symptoms
In infected plants the upper surfaces of affected leaves have ill defined, irregular, yellowed areas, covered in fine black dots, while the lower surfaces will have lesions which are covered in white to grey sporulating fungal growth.
Development
Air-borne spores spread the disease to healthy plants throughout the growing season. Lesions primarily develop on the leaves, but stems and pods may also become infected.
Infection can be introduced by either airborne spores from volunteers and nearby brassicas or oospores produced by previous crops which are able to survive in the soil without a host. The spores germinate by producing germ-tubes that penetrate cell walls, often on the lower side of the leaves, and then the leaf tissue becomes colonised.
Downy mildew can occur on leaves throughout the year and often spreads through the canopy during stem extension and flowering. It can also infect pods which will be covered in yellow or fawny brown lesions. In humid conditions these are covered in white to grey sporulating growth. If the infection is severe enough the whole pod will bronze and ripen prematurely.
Favourable Factors
Infection is favoured by temperatures between 10 and 15°C and humidity from rain or dew. Late emerging seedlings are more at risk.
Importance
Although common in wet autumns, the disease is generally of little economic importance. Severe infections may result in the premature loss of cotyledons or leaves during frosty weather. Later sown or backward crops can be more at risk, particularly in wet conditions.
This disease is a threat to OSR crops from emergence up to the three true leaf stage.
Control
Avoid late drilling, early drilled crops are able to grow relatively quickly through the susceptible stages and so are at less risk.
Fungicidal seed treatments can protect emerging crops from early attacks of downy mildew.
At the time of writing foliar fungicides options for downy mildew control in OSR are very limited compared to previous years.
