This morning I found a white powdery substance on my cut mums. After dispelling my initial thoughts of a plant party (When flowers go WILD!), my next suspicion was powdery mildew. Fortunately, Peter discovered that is was merely the residue of a pesticide spray. Had it been powdery mildew it would not have been so widespread. Infections tend to have a single point of origin, Peter told me, they don't start spread out across a number of different plants and cultivars.