Powdery mildew on plum isn't a common issue in this region. If you could send me some photos or a description of the symptoms, then I will take a look at it and confirm whether it is in fact powdery mildew. If it is powdery mildew, then you can apply a sulfur spray when the tree is in full bloom. Keep in mind that bees will be actively foraging on flowers, so you should only spray if needed, and spray in the evening after dusk when the bees are less active to allow the spray some time to dry overnight.