Pm is not a systemic fungal disease. Eagle 20 is a systemic fungicide which means it is xylem/phloem mobile through out the plant and protects old and new tissue because of this. PM does not spread though out the plant via the xylem/phloem only superficially on the leaf surface. There are fungi that are systemic in plants, both parasitic and beneficial, but there are no known Erysiphales spp. that are systemic. PM starts first by spores, in our case conidia, that germinate on the leaf surface. Mycelium spreads across the leaf surface and inserts cell structures through the epidermis of the plant called haustoria. These cells sap nutrition from the plant and is the reason you see yellow spots after its been "washed" away by PM wash. This may be where the stoner misconception of "systemic" comes from. Even though the haustoria is inside the plant the organism itself does not emerge nor can it survive/reproduce inside of the plant, it needs its reproductive and vegetative structures that exist only on the leaf surface. PM is considers an obligate biotroph.
PM is very difficult to fully eradicate from an indoor environment and is another reason I think a lot of people think it is systemic. Its hard to ensure that all conidia have been destroyed in the environment and they cause reinfection if they are not. When you see a spot of PM on a plant you are actually looking at millions of tiny conidia stacked up in chains off the leaf surface. If you see just one spot of PM anywhere on a plant you can be sure there are are shit load of spores in the environment and it will only be a few weeks before you start seeing it everywhere again.
The link Vertigo was so kind to post up is a copy of Hemp Diseases and Pests by McPartland, Rob Clark and Dave Watson (better know as Sam the Skunkman) that I sent him. If you dont own a copy I suggest you all download and take a peek. Its time to put down Jorge Cervantes and Ed Rosenthols "Grow Bible" and step out of the stoner misinfo soup these forums seem to cook up. One of the first things you may notice if you go to the PM section is that there are actually two species of fungi that attack cannabis as "Powdery Mildew" which is a catch all for thousands of species that cause the same disease. Interestingly enough one is able to be controlled via the environment(RH and Temp) where as the other can germinate at 0% RH and High temps. Probably why some have such great luck with a simple change of environment and a systemic treatment such as E20, Quadris, etc.
My $.02
