Downy Mildew is a very common disease of roses which occurs during periods of moderate temperatures and high humidity.
The disease is extremely contagious and will spread throughout the rose greenhouse very quickly if left untreated, laying waste to all the plants within a very short time.
Once it takes hold, it will defoliate a plant very rapidly.
While total defoliation does not immediately kill a plant, its loss of photosynthesizing ability stresses and weakens the plant to a degree that it becomes totally unproductive, a situation from which it may never recover.
- Leaves have reddish-black spots that are “angular” – having a flat side. When a spot comes to a leaf vein, it follows it.
- Advanced infections will have yellowing of leaves with brown necrotic areas and noticeable leaf abscission.
- Plants are rapidly defoliated, usually from the top down.
This disease can be especially severe on greenhouse grown roses where humid conditions exist for long time periods. Ventilation and reducing humidity below 85% will reduce disease development. Sanitation in the greenhouse will reduce the primary source of inoculum.
Where damp humid conditions exist, systemic fungicides will give some control.
Other fungicides can also be used but the persistence of the material through wet periods or rain is important to maintain protection.