I have been trying all summer to get cuttings from a flowering cherry. Every time I do, I follow the simple procedure of cutting, adding hormone and placing in substrate with some moisture in a bright, but not direct sun spot inside large ziploc bags. But every time after a couple days they have lots of fluffy cottony white fungus on them (powdery mildew?) What is the fix for this? Do I just grab an off the shelf fungicide and just spray them and seal the bags back up?