We have powdery mildew-a microscopic fungal infection in both of our rose plants, and the veins did not turn yellow, rather the leaves had a splotchy or freckled appearance and also a rather shriveled look when they unfurled. If left they will turn brown, but they usually don't last that long as the lightest touch causes the leaves-sometimes entire shoots- to come right off. Even in the wind.
Not the best news in the world. I don't think my miniatures are going to make it.

I am no expert, however, so I may be wrong, though it does sound like what happened to our plants. On one plant this wasn't evident until I treated the leaves with an oil to smother the fungus, but on the other plant I noticed it before treating it with anything.

Shawna
From the Heart of the City