You don't describe any other symptoms, or varieties, so it's hard to know. It could be black spot, the nemesis of many a rose. It really hits hard in the hot, humid days of summer and causes partial to complete defoliation. There are other diseases that can defoliate as well. One of the things that is so great about many modern rose varieties is they have better resistance to black spot as well as other diseases, so you don't have to spray fungicides to keep the leaves on your plants. I'm sure you don't like the idea of tearing out your roses to put in new ones, but these are the options: put up with things as they are, pull out the roses and replace with newer, more disease resistant types, or put your roses on a fungicide spray. Any garden center should carry the appropriate fungicides. If you would like to keep your roses, a few things might reduce the symptoms: all infected leaves and prunings should be discarded or burned, not composted. Also, avoid overheard watering, which spreads the disease around. Resistant climbers include Altissimo, Golden showers, Joseph's Coat, and Royal sunset. David Austin ramblers include some reliable varieties too.