This is black spot, a fungal disease particularly common to roses, caused by the organism Diplocarpon rosae. It's a summer problem: the fungus needs at least 7 hours of temperatures above 74° and moisture. Does that sound like Houston or what? I think we are in for another 3 months of temperatures above 74° and the humidity here is enough to cause fungal problems on inanimate objects.
Black spot is a problem for roses because eventually these leaves all turn yellow and drop. A bad infection can cause defoliation of the entire plant. The disease is most pronounced on new leaves, and young plants can be killed outright.
Fighting black spot requires good cultural habits. Try not to get water on the leaves of the roses, and space them far enough apart to ensure good air circulation. Remove diseased leaves from the plants and keep the fallen ones raked up and out of the compost pile. The organism that causes black spot can overwinter in garden debris and jump right back up when temperatures warm up. Select resistant varieties if you can -- more and more rose breeding is focused on disease resistance.
There are conventional fungicides that can be applied to control black spot: Synthetic protectant-type fungicides can be used in small quantities and don't appear to contribute to the problem of resistant disease organisms. Systemic fungicides tend to promote resistant diseases and often negatively affect soil micro-organisms. The most frequently recommended organic solution is sulfur, which has problems of its own. Yes, it's organic and yes, it's been used for thousands of years. But it only works while a film of sulfur remains on the leaf and thus must be frequently reapplied. Sulfur also tends to build up in the soil, and can burn plants if applied when the weather is hot (like it is when black spot is a problem). It can also cause problems with non-pest and beneficial insects. And sulfur mining's not the most environmentally friendly thing you can do either!
So we're left with disease-resistant roses. Bear in mind that disease resistance doesn't necessarily mean disease-proof. I've seen black spot on Knockout roses and every other kind of resistant rose. The strong ones fight it off, but if you're looking for picture-perfect foliage, skip the roses. In our climate, you'll be resorting to some pretty heavy chemical weaponry to maintain perfection.
Here are some great pictures of sulfur mining and here are the rose cultivars that have been awarded Earth-Kind status, in part because they are resistant (not immune) to black spot.
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