If your rose leaves turn yellow, have black sooty spots on them, die and fall off, then they have black spot, which is a fungal disease. Prevention of this is to buy resistant roses, which I did, and try not to get your roses wet while watering. Also don't use foliar sprays (spray on fertilizer) on roses that are susceptible to black spot.
We have had some very wet and cool weather for this part of the country lately and I foolishly sprayed my roses with foliar spray in the midst of all this. One set of roses looks great and very healthy, the other set have lost most of their leaves. Maybe it wasn't my fault, because my sister's roses are also afflicted, poor dears.
This is the spray I am using on them to kill it. I'll let you know how it goes : )
To rid roses of black spot, mix:
1 gallon of water
1 tablespoon baking soda
1 tablespoon vegetable oil
1 tablespoon dish detergent
Spray once a week.
I doubt I will spray once a week all summer, knowing me, but at least for a while until they recover.