Treatment of downy mildew is only likely to be warranted on younger crops. Treatment of phoma or LLS may be warranted if at least 25% of plants have 20% infection. This threshold should be higher when plants are bigger and possibly vice versa.
Products like Folicur, Fexan, Proline, Magnello or Prosaro might be used. Some of these also act as growth regulators which might be useful for very advanced crops but not on more backward ones. The PGR effect is also rate related. The minimum rate for disease control will be half label rate and higher rates give more growth regulation where needed.
Tillage management: Good week sees planting progress
Tillage management: Tips for winter planting
Tillage management: Aphicide and herbicide needed in many cases