Blackspot is spread by rain and by overhead watering, so the irrigation system is likely spreading it all over the rose bushes that are within their zones. You didn't say where you are located, so I don't know what your winters are like, but you can strip all the leaves off this fall after the first hard freezes, if you get them. Rose canes typically lose their leaves during cold winters, and the canes are normally pruned back in the very late winter or early spring, which prompts healthy new canes and leaves to grow. Rose bushes continually replace older canes with new ones.
You may want to try to save the rose with the infected canes by simply cutting them off an inch above the ground. The rose will then replace those canes with healthy new ones. You may want to reconsider which fungicides you use, and how often you apply them, if the roses are sprayed by the irrigation system on a daily basis, or several times a week.
