That is not powdery mildew, just water spots. Nothing to worry about. Notice how the marks are concentrated around the edges of the leaf? At the tip of each of the serrations of the leaf there is a vein, which ends in something called a hydathode, which will put out a drop of water + nutrients in the early morning when the humidity is high. That drop will dry, leaving the nutrients behind as salt. The salt usually means you might be fertilizing too much.