Actinovate to Prevent Powdery Mildew

This evening I sprayed the pumpkin plants with Actinovate to help slow down powdery mildew which is starting to show up.  At this point of the season I'm not overly worried about it, but I don't want to let it get out of control.  The 335 plant has shown the worst early signs of it but it is okay.  In the last few days I've seen it show up on the 1421 plants older leaves that are shaded.  Strangely the 1775 plant has shown none.  What is strange about that is that with the leaf burn this plant has sustained you would think it would be more suspectible but I haven't seen any spots on it so far.