For non-rosarian readers, I should explain that the disease that troubled my Abraham Darby rose bush (see my last post) was rust. The cloud of orange dust around the poor bush came from the fungal spores that spread the disease.
The big three rose diseases are blackspot, rust and powdery mildew (downy mildew is a more serious rose disease but is thankfully rare and so isn't included in the short list). All of these are caused by fungi. Blackspot (BS) and rust spores need 4-5 hours of moisture to begin growth, so they're more of a problem in humid and/or rainy climates than in places like Spokane. Powdery mildew shows up when days are warm and nights are cool, and it doesn't need moisture to germinate. I see it around here in late summer and fall.
Dee at Red Dirt Ramblings just posted a great treatise on blackspot control, and many of her practices will help with rust control as well. The best prevention for powdery mildew seems to be ample water, as water-stressed plants are more susceptible to the disease. I'm not well versed in all the organic and non-organic fungicides out there, but I like Dee's suggestion to start with the least toxic treatments first.
In his book about English roses, David Austin noted that rose-growers of yesteryear used to get a sense of satisfaction from all the spraying required for healthy rose bushes. Gardeners nowadays are wiser or lazier or both and don't want to deal with the sprays, so disease-resistance has become an important aim for hybridizers. Of the english roses, 'The Mayflower' is advertised as being completely resistant to the big three rose diseases, and most of the newer english roses are listed as highly resistant. After my Darby debacle, disease-resistance is a priority for me when selecting new roses.
Added January 15, 2015 - I decided to give Abraham Darby another try a few years ago, and I haven't seen a speck of rust on him here in Spokane. I'm so happy to once again enjoy his sumptuous flowers and citrus scent in the garden and in the vase.