So if there is no improvement in disease development, resistance may be a factor and use a different product. For the most part, fungicides act as a preventative. They prevent infections from occurring, they do not get rid of existing infections. The white to gray powdery growth on the leaf will remain even after fungicide use. In addition, repeated applications are necessary to control powdery mildew.
Downy mildew is very different from powdery mildew.
Downy mildew is in the same class of fungi as the root rotters Pythium and Phytophthora and is considered a water-mold. This is why the cultural controls and fungicides to control powdery mildew are ineffective on downy mildew. Downy mildew on flowering potted plants is relatively rare. It is much more common on perennial and garden plants such as Coreopsis, Centaurea, Rudbeckia, Pelargonium, pansy, and roses. Downy mildew produces its characteristic symptoms on the leaf underside.