Early this past autumn I introduced into my garden
one rose (it's still in a pot, as are many of mine even
now) with a little powdery mildew on a few leaves.
I halfheartedly sprayed it with the usual diluted
peroxide I use to prevent black spot from spreading,
figuring that that would at least kill any spores and
(with luck) prevent any spread. No more leaves
were affected and I nearly forgot all about it.

Months of dry weather and high humidity later: about
a dozen roses have powdery mildew. I had never seen
it on my land befo apparently that one rose was enough
to get it established. I've just pruned off the curled-up
leaves and applied the fungicidal spray.

I must note that most of the roses afflicted with it were near
the infected plant, and that they are old varieties that are
perhaps more prone to it than newer ones (I haven't checked
any reference books, such as Beales', which note that certain
cultivars are prone to mildew, black spot, or whatever).
Still, this goes to show that it pays to be cautious, and perhaps
more aggressive with the fungicides.

Mark., now why isn't that grasshopper disease helping things
more? And the deer repellent?