Sorry I didn't get back sooner. Yes, that looks like powdery mildew. Normally a couple fungicide applications will do it. Sovran, Flint, Rally Rubigan, Inspire Super, or Topsin M will all work. Most are pretty expensive though, Topsin M should be the cheapest. Not sure you can get them in small quantities though or if they are restricted use in your state. Check around the web. Good luck.
Oh, were they in tree tubes or wrapped in something?