Hi AMonell,
Welcome to this National Gardening Association forum. I don't have a greenhouse, but I do have an indoor zinnia project that involves many of the same problems regarding mildew. You could use Physan 20.
http://www.physan.com/
I have used quite a lot of it for many things, including killing algae growth in my potted zinnias.
ZM (not associated with any product or vendor mentioned or linked)
