How do I control powdery mildew and black spot in polyhouse Dutch rose?

Powdery mildew shows as white powder mainly in cool nights with warm days and high humidity swings, while black spot shows as dark spots with yellowing and leaf drop in wet leaf conditions. Both are managed first by environment, keep humidity controlled, ensure air movement in the polyhouse, avoid wetting the foliage, and remove and destroy fallen infected leaves since the disease carries over on them. Good spacing and not overcrowding helps air flow. For sprays, use only fungicides labelled for rose and the specific disease, follow the label dose and waiting period, and rotate modes of action to avoid resistance, which builds up fast in rose. Sulphur based and other approved fungicides are commonly used but must be rotated. Because resistance and residue both matter, get a current fungicide rotation programme from your KVK or a floriculture specialist rather than spraying one product repeatedly.