There is an interesting post over at Treehugger (previously known to me only as a place to find cool eco-friendly stuff to buy) about one weed's anti-fungal capabilities. Turns out that extracts from ground up and dried Giant knotweed (Fallopia sachalinensis, Polygonum sachalinense, Reynoutria sachalinensis, etc.) have been found to very effective at preventing a variety of fungal infections, including the insidious powdery mildew. Which means, of course, that a company has come along that would like to spray it all over everything (Read the EPA docs here). The product, Milsana® Bioprotectant Concentrate, has been under development for quite a few years, but while I found this label (.pdf), I can't seem to find any place to buy it - perhaps it is intended for commercial use only.
Thanks to Al over at Urban Wilderness for sending in a link to this story.