Powdery mildew usually occurs when the plant does not get enough air moving through the plant material and mildew occurs.  You can try to thin out these plants by trimming some of the branches and these cool nights may be getting cool enough to cause some moisture to linger on the leaves.  There are some spray products to apply that may mitigate this but getting more air through the plant will be the way to solve this for this fall.  Is there lawn sprinklers that are over spraying this area in addition to the drip irrigation????